MEXICO CITY, Dec. 27 -- Hoping to build a new bulwark against the flow of illegal drugs from Latin America, the United States is providing the Mexican military with extensive covert intelligence support and training hundreds of its officers to help shape a network of anti-drug troops around the country, United States and Mexican officials say.

The officials say the assistance has included training, equipment and advice from the Central Intelligence Agency to establish an elite army intelligence unit that has quietly moved to the forefront of Mexico's anti-drug effort, sometimes ahead of a new civilian police force that the United States is also pledged to support.

The effort has proceeded despite growing American concern that it may lead to more serious problems of corruption and human rights in one of Mexico's most respected institutions, United States officials say. A new United States intelligence analysis of the military's drug ties, for instance, will cite evidence of extensive penetration of the officer corps, two people who have seen draft versions of the assessment said.

Clinton Administration officials have described the American aid as a stopgap. Echoing Mexico's President, Ernesto Zedillo, they insist that the military's law-enforcement actions will be limited and temporary, helping to disrupt the country's thriving drug trade only until its badly corrupted federal police forces can be overhauled.

But according to many officials, the Pentagon and the C.I.A. have pressed their help partly out of their need to find new tasks after the cold war. They hope to use the aid to expand their roles in the anti-drug campaign in Mexico and to improve their relationships with a secretive, nationalistic neighboring army that has often looked at them with suspicion, the officials said.

''They didn't have anybody to play with on the Mexican end of the drug issue, so they went for the military,'' a former senior official who was involved in American policy in Mexico said, referring to the Defense Department and the C.I.A. ''They knew the risks, but they thought they could control the situation.''

Some of those risks have resounded in recent news reports: the jailing of army generals on charges of protecting major drug traffickers; allegations that military officers have been linked to the torture and disappearance of criminal suspects; failures of due process and proper legal procedure by soldiers stepping in for the police.

Other pitfalls have been less apparent. Some officials, for instance, worry that American intelligence officers may face conflicts in trying to build good relationships with Mexican Army officers to sustain the cooperation, and trying to remain watchful of military corruption at the same time.

A few other current and former United States officials date their unease to what they described as a disastrous C.I.A. program in the late 1980's to deploy a Mexican Army strike force against the traffickers. The force was disbanded after several failed operations, one of which resulted in the killing of four Mexican civilians.

Mexico has long stood out in Latin America for the sureness of its civilian control over the military. But American officials said they had been troubled by indications that some officers detailed to the federal police have operated with considerable independence from the judicial authorities. With the Mexican Army searching for new missions, many American officials doubt that it will limit its participation in law enforcement to the two-year deadline that Mr. Zedillo and his aides set last summer.

''The whole thing has snowballed,'' said a senior American official who, like others, would discuss the matter only on the condition that he not be identified. ''We are now seeing two separate anti-drug efforts in Mexico -- one by the military and one under the Attorney General. If I were in the Attorney General's office, I would be asking whether it has gone too far.''

To some degree, the policy debate is fueled by old rivalries between American law-enforcement agencies and their intelligence and military counterparts. But the two sides also have some philosophical differences, which center on the question of whether United States support for the military complements or competes with efforts to transform Mexico's crippled criminal-justice system.

''They have basically got to rebuild their entire police force,'' a senior drug-enforcement official said of the Mexican Government. ''You can't do that in a year or two. And the longer the emphasis is put on the military, the longer it is going to take to get the police up and running.''

The Pentagon spokesman, Kenneth H. Bacon, disputed the idea that the two directions of American anti-drug aid in Mexico were at cross purposes. ''There is no conflict,'' he said.

James R. Jones, who left Mexico City this summer after serving for four years as the United States Ambassador there, echoed that view, and denied that American officials had encouraged the Mexican military's new role.

''The temporary detailing of military officers to civilian law-enforcement was the Mexicans' and Zedillo's decision -- we had nothing to do with that,'' Mr. Jones said. ''Our efforts to improve the quality and exchange of intelligence information and our training programs for certain military units had nothing to do with their decision.''

The C.I.A.'s chief spokesman, Bill Harlow, declined to comment on the agency's activities in Mexico.

Past Experience: Corruption Seen In Old Program

Despite a widespread belief in the corruption of Mexico's federal and state police, Mexicans and their political leaders have been wary about seeking the help of the military to enforce civilian laws.

In the late 1970's, Mexican officials turned to the army to help drive marijuana and heroin-poppy growers from their sanctuaries in the rugged folds of the western Sierra Madre. Their sweep succeeded in temporarily dislodging the traffickers, and it institutionalized a program in which about 20,000 of the army's 150,000 soldiers are detailed to drug-crop eradication campaigns.

For an army that has relatively little to do in securing the country's borders, the drug eradication program has been a source of pride. Yet even while it avoided police-type activities, the military was shaken during the 1980's and early 1990's by public allegations that some senior officers -- including a former Defense Minister, a Secretary of the Navy and several senior army commanders -- colluded in the drug trade.

After Carlos Salinas de Gortari became President in late 1988, American officials said, they gave relatively little thought to the Mexican Army because of their hopes that at least one of the Government's many police-reform campaigns would succeed. Some also harbored cause for concern in the experience of the Mexican Army strike force created years earlier.

With anti-drug efforts stalled in 1987, American agents developed information that big traffickers were building nearly impenetrable compounds in the countryside. Working with the military, current and former officials said, C.I.A. officers helped form what they envisioned as an elite team of about 50 soldiers that would strike more effectively and operate more securely than the police. Mexican law-enforcement officials were told nothing of the plan, they said.

The team's first operation, against a stronghold of a cocaine smuggler in Sinaloa state, ended with one soldier's capture by a police agent working for the traffickers, two former officials said.

The next foray went considerably worse. On the morning of April 11, 1988, helicopters swung out of the dawn sky near the northern town of Caborca, a sanctuary of a reputed marijuana smuggler.

''The idea was that you could take a well-trained military unit and go in there and boom -- take everybody out,'' a former official said.

The soldiers did take everyone out, but they did so at what turned out to be a workshop in a residential neighborhood, killing four apprentice welders. Former officials said the Mexican Attorney General at the time, Sergio Garcia Ramirez, was so mystified that he asked whether American agents had carried out the raid themselves.

Eventually, the military issued a terse statement taking responsibility for the attack but not disclosing the C.I.A.'s involvement. After a third, unsuccessful raid on another suspected drug base, the program was shut down for good, the officials said.

Zedillo's Effort: Military Is Given A Bigger Role

Mexico was near the same point in its political cycle -- the end of a six-year presidential term, a period when corruption has historically flourished among outgoing officials -- when United States officials looked to the military again in 1994.

The idea of greater army support for the police was raised first by American diplomats and again during a visit by the United States Army Chief of Staff, Gen. Gordon Sullivan, current and former American officials said. But at the time, they said, it was rejected by Mexican justice officials.

Mr. Zedillo, who took office on Dec. 1, 1994, has called drug trafficking one of Mexico's most serious problems of national security. United States officials strongly endorsed that view, briefing his aides on such developments as the use of passenger jets to fly cocaine into Mexico from Colombia.

''The military was the only trained, disciplined force that you could use to deal with this situation in the short term,'' one of Mr. Zedillo's closest aides said. ''There was no one else.''

Mr. Zedillo first brought army commanders into the redesign of the Government's drug-control strategy. He then authorized them to work with United States officials in an ultimately abortive effort to deploy its aging F-5 fighters to chase drug jets. Finally, he began allowing military officers to replace federal police agents in several border cities plagued by smugglers.

In October 1995, when William J. Perry made the first official visit to Mexico in memory by an American Secretary of Defense, anti-drug aid was at the center of several cooperative ventures he proposed to Mexican military officials, Mexican and United States military officials said.

''You were looking for general ways to engage, military to military,'' a Pentagon official said. Within months, a first group of young Mexican Army officers were training in anti-drug operations at Fort Bragg, N.C.

Of some 3,000 Mexican soldiers who are expected to have passed through Defense Department training courses by next fall, 328 young officers will have completed special 12- and 13-week programs intended to create a corps of anti-drug specialists. Those trainers are being sent in turn to train air-mobile special forces units that are now stationed at the headquarters of the 12 regions and 40 zones that make up Mexico's military geography.

Defense Department officials said the anti-drug curriculum of the units, called Air-Mobile Special Forces Groups, ranged from air-assault operations and military policing to human rights. The Pentagon has also given Mexico 73 aging UH-1H helicopters to transport those troops.

The helicopters may be used only for anti-drug operations. But Mexican and United States military officials said there was nothing to stop the transfer of American-trained army officers to similar special forces units that might be deployed against leftist insurgents in southern states like Guerrero and Chiapas.

C.I.A. Support: Special Force Gets Mixed Reviews

American officials said that what is perhaps the most significant United States support for the Mexican military's anti-drug efforts is probably the least visible. It comes, they said, in the training, equipping and operational support of C.I.A. officers for a special force of the army intelligence section called the Center for Anti-Narcotics Investigations.

The unit, comprising some 90 carefully chosen young officers, began to come together about three years ago, officials said. Like the civilian intelligence groups the C.I.A. works with in Mexico, the military anti-drug force is not supposed to be an ''action'' unit like the group trained by the agency in the 1980's. But it does appear to sometimes take the lead in raids as well as surveillance actions.

Several American officials compared the program to the C.I.A.'s work in Colombia, where the agency has been credited with critical help in the capture of major drug traffickers. A key difference, they noted, has been Mexico's extreme sensitivity to anything involving the C.I.A.

Officials familiar with the operations of the intelligence team said that after a clumsy start -- at one point its agents lost track of an important Bolivian drug broker they had under surveillance because they insisted on asking a superior for instructions rather than simply follow him -- it has emerged as probably the most active of all Mexican anti-drug units.

Officials said the unit played a central part in the pursuit of Amado Carrillo Fuentes, then Mexico's most important trafficker, before he died during plastic surgery last summer. It also worked closely on the investigation of Mr. Carrillo Fuentes's organization after his death, and on a series of raids against the Tijuana-based drug mafia run by the Arellano Felix brothers.

Yet reviews of the unit, which is known by its initials in Spanish as the Cian, have been mixed.

Officials said some Mexican prosecutors have complained privately that the unit's officials have demonstrated spotty notions of the law, at times handing captured suspects over to the civilian authorities without ever gathering evidence to hold them. Some Mexican police investigators have also questioned why -- if the United States is willing to provide the sort of sophisticated surveillance and intelligence-gathering equipment that it is said to have given to the Cian -- it will not offer the same support to new anti-drug units created in the Attorney General's office.

American officials said questions had been raised about the unit's integrity after two of its agents were dismissed this year for what one official described as ''unprofessional conduct.'' Some have also wondered about its independence -- from both Mexican civilians and American intelligence officers.

''It could be a time bomb,'' a former intelligence official said, ''because they have a lot less control over that unit than they think they do.''

The Mexican Army's Chief of Staff, Gen. Juan Heriberto Salinas Altes, praised the unit, saying it has gathered important information for both army special-forces troops and the federal police. He denied, however, that the unit has any formal or continuing relationship with the C.I.A.

''There could have been some contact, but it was not any official contact,'' General Salinas Altes said in an interview, his first since becoming Chief of Staff three years ago.

Vulnerability: Can the Military Resist Temptation?

For their part, Clinton Administration officials said the closer military relationship that anti-drug cooperation had already paid dividends for the United States. In recent months, they noted, Mexican officials agreed to streamline procedures by which United States drug-surveillance planes are allowed to fly over Mexican airspace, and those by which Coast Guard ships can dock at Mexican ports.

The impact of American support on the Mexican military's anti-drug efforts remains somewhat to be seen. Thus far, Defense Department officials said they knew of no instance in which special forces officers trained in the United States had been sent off on an American-donated helicopter in the pursuit of a drug flight.

But many United States officials said it had already become evident that although the Mexican officer corps may be more resistant to the traffickers' bribes than the police, it faces a more serious threat than most American officials foresaw.

Clinton Administration officials were shocked this year when the army commander installed as the Zedillo government's drug-enforcement chief, Gen. Jesus Gutierrez Rebollo, was arrested for taking bribes from Mr. Carrillo Fuentes, the trafficker. Since then, though, it has become clear that the episode was not an isolated one.

Other high-ranking officers have been implicated in connection with Mr. Carrillo Fuentes's organization, including one retired general who was arrested after the wedding of the trafficker's sister. Several more senior officers have been arrested for their supposed ties to the rival trafficking organization run out of Tijuana by the Arellano Felix brothers; one has been charged with offering $1 million monthly bribes to another army general who serves as the Attorney General's representative there.

Still, United States officials are divided between those who see new proof of the military's vulnerability and those who see evidence of an institution fighting aggressively against temptation. Mexican officials, not surprisingly, side strongly with the latter camp.

''If there is action, there are going to be people hurt,'' General Salinas Altes said. ''We have people killed. We have people wounded. We have people in jail.''

General Salinas Altes has himself been a focus of the American scrutiny.

According to officials familiar with American intelligence reports in which the Chief of Staff is mentioned, he first came to the United States' attention after he moved from Baja California in 1988 to head the Ninth Military Region headquarters in the city of Acapulco. There, he spent six years in charge of drug-eradication efforts in Guerrero, the state that is Mexico's leading producer of heroin poppies.

After he became Chief of Staff in December 1994, two American officials said, General Salinas Altes was again briefly a subject of scrutiny when intelligence officials intercepted a drug trafficker's telephone call for a ''General Salinas.'' They described the report as disturbing but unclear.

Most recently, United States officials became concerned again this summer when it emerged that General Salinas Altes and several other senior officers had met with a top lieutenant of Mr. Carrillo Fuentes. According to a military document published in the Mexican magazine Proceso that appears to be notes from the Jan. 14 meeting, the lieutenant, Eduardo Gonzalez Quirarte, said the trafficker had offered essentially to clean up his business -- halting the sale of drugs in Mexico, eschewing violence, helping the economy -- if he was allowed to keep half his fortune and continue operating in peace.

General Salinas Altes was interviewed at his Mexico City offices on the condition that he not be questioned about the allegations against him, but only about the cooperative efforts he oversees as the military's representative to an anti-drug group of senior officials from the two countries.

But in a separate interview, Gen. Tomas Angeles Dauahare, a senior aide to Mexico's Defense Minister, Gen. Enrique Cervantes Aguirre, dismissed the allegations against General Salinas Altes vehemently.

General Angeles confirmed that Mexican military officials had received an unsigned American report forwarded from the Foreign Ministry. After an extensive investigation, in which United States officials did not answer repeated requests for supplementary information, he said, it was found that the guilty officer was in fact a Gen. Javier Salinas Payares, the commander of a military air base, and that he was eventually imprisoned in the case.

Similarly, General Angeles said there was no evidence that General Salinas Altes had acted inappropriately in meeting with Mr. Gonzalez Quirarte, who he said had posed as a young businessman with information about the Carrillo Fuentes mob.

''He spoke about Amado Carrillo,'' General Angeles said. ''He gave information about the drug organization internationally. It was only discovered later that this was Eduardo Gonzalez Quirarte.''

General Angeles also strongly denied a claim made by General Gutierrez Rebollo during his trial that Mr. Gonzalez Quirarte had two other meetings and that he paid a $6 million down payment on a promised gratuity of $60 million to Government officials.

''You can be fully certain that had he done that, he would have been arrested because that is bribery,'' General Angeles said. Referring to General Gutierrez Rebollo, he added, ''That is characteristic of his accusations -- the lies, the infamy, the slander.''

One current and one former American official said there was credible information that Mr. Gonzalez Quirarte had been to see military officials more than once, and they added that a recent assessment of General Gutierrez Rebollo's testimony concluded that much of it was true.