The US newspaper Newsday said 60 Russian irregulars joined Serb paramilitaries in the killing of hundreds of people around the city of Prizren.

The newspaper cited ethnic Albanians and a Serb security force source in its report.

The US defence department said some Russians had fought alongside Serbs, but that there was no "firm evidence" on their numbers or where they operated.

Pentagon spokesman Ken Bacon said: "We certainly know that Russians participated.

More than 100 mass graves have been reported in Kosovo

"Our impression is that they were mercenaries, but this is something that I'm sure will be looked into by war crimes investigators, who are now going into Kosovo in considerable numbers to try to figure out what happened and who was responsible for it."

Newsday reported that the Russians, who were retired or not on active military service, were organised as a single unit under the Yugoslav Interior Ministry's Special Purpose Police and commanded by an army colonel.

Russia's role in Kosovo is a sensitive issue after Moscow opposed the Nato bombings and because of pan-Slav cultural ties.

An agreement on the role of Russian troops in the peacekeeping force was reached over the weekend after days of vexed negotiations.

"When the Russian forces arrive as part of KFOR, we expect them to be totally fair and professional in their dealings with both Serbs and Kosovar Albanians, as they have been in Bosnia and where they have been very stalwart and successful members of the peacekeeping force," said Mr Bacon.