President Slobodan Milosevic, who will be in Ohio next week for US-backed peace talks, may be behind a major war crime.

Muslim witnesses say that an officer from Serbia was directing the roundup of Muslim prisoners in the village of Konjevic Polje, and that a Serb officer captured by Muslim forces was following orders issued from the Serbian capital, Belgrade.

''[The Serb officer] said they were under orders from Belgrade not to allow any men to escape from Srebrenica,'' says Bosnian soldier Dzemal Malovic.

''All Muslim men were to be captured or killed,'' said Mr. Malovic, one of three Bosnian soldiers who say they spoke to and looked at identity papers of the captured Serbian captain.

In a separate interview, a Muslim officer confirmed that the Serbian officer had been captured. The Serbian officer's whereabouts are unknown, and he may have been killed later by Muslim forces.

Western diplomats have long suspected that the Bosnian Serb attack on Srebrenica in mid-July was approved by Belgrade, but the government of President Slobodan Milosevic has vehemently denied it.

Mr. Milosevic's involvement would be an embarrassment for the Clinton administration, eager to portray Milosevic - who will be attending peace talks in Ohio next week - as a peacemaker in the Balkans, not a war criminal.

''Whether by commission or omission, [Milosevic] is responsible, no question,'' says a senior UN military official based in Zagreb, Croatia. ''He had plenty of sources on the ground there. He had to know what was happening, and either approved of it or did nothing to stop it.''

A senior Western diplomat in Zagreb also says Milosevic is responsible for what is quickly emerging as one of the darkest hours of Bosnia's 3-1/2 year conflict.

''I have no doubt he directly approved or tacitly approved of the taking of Srebrenica,'' the diplomat says. ''Whether Milosevic knew [about the executions] or not, he knows what kind of man [Bosnian Serb Army commander Gen. Ratko] Mladic is and how he operates.''

Reports of Mass Executions

Over 2,000 Muslim men were executed by Bosnian Serb forces following the fall of Srebrenica, according to nine survivors interviewed by the Monitor last month.

War-crimes investigators now have evidence that as many as 3,000 to 4,000 men were executed by the Bosnian Serbs, according to a senior UN official close to the investigation. ''Wait until everything comes out,'' he says. ''Then, people will understand how big this is.''

The UN official close to the International War Crimes Tribunal in The Hague said mass graves ring the area around Srebrenica, and confirmed the existence of a new set of United States spy photos showing a new group of apparent mass graves near the village of Karakaj, as reported by the Boston Globe on Oct.3.

The photos confirm the accounts of five men interviewed by the Monitor who say over 2,000 Muslim prisoners were executed near the town of Karakaj on July 18. The photos may be the basis for new indictments against General Mladic expected to be issued by the Tribunal.

Bosnian Serb officials have said that mass graves in the village of Nova Kasaba captured in US spy photos and visited by the Monitor in August contain the bodies of Muslim soldiers who were killed in combat and not executed.

But the Karakaj site is too far from the route that Muslim men would have followed to escape from Srebrenica, according to the UN official.

Mevludin Oric, a survivor of the Karakaj execution, said in an interview that one of the officers directing the roundup of prisoners in Konjevic Polje was a 40- to 45-year-old officer from Serbia. Mr. Oric is considered by war-crimes investigators to be one of their most credible witnesses. The Serbian officer was not present at the later execution, Oric added.

Who gave the order to execute thousands of prisoners remains unknown.

But evidence of Milosevic's involvement in Srebrenica has been mounting for months, and it is not known if Mladic would execute such a large number of men without at least the tacit approval of Serbian leaders in Belgrade.

Mladic, who eyewitnesses interviewed by the Monitor said was at Karakaj and three other executions sites during or just before executions began, had been visiting Belgrade regularly for weeks prior to the attack.

Dutch peacekeepers reported seeing members of paramilitary groups from Serbia, and Muslims say they saw Belgrade-based paramilitary leader Zeljko ''Arkan'' Raznjatovic in Srebrenica.

The Washington Post reported seeing Muslim soldiers driving a jeep with Yugoslav Army license plates on July 17. The Muslims said they had captured the jeep from forces involved in the attack on Srebrenica.

New York Newsday reported on Aug. 12 that Western intelligence officials captured radio intercepts of Yugoslav Army chief Gen. Momcilo Perisic, directing Mladic on how to attack Srebrenica during the offensive.

Serb Denials

Yugoslav officials have strenuously denied the accounts, but the Yugoslav Army and Arkan are believed to be tightly controlled by Milosevic, who holds an iron grip over Serbia's military.

Despite the growing evidence, Srebrenica survivors remain skeptical that Milosevic - whom the Clinton administration is depending on to force the Bosnian Serbs to agree to a peace deal - will be tied to or punished for Europe's worst massacre since World War II.

''It all depends on the politicians,'' Malovic says. ''They could punish them, or reward them, for doing this.''