Carla del Ponte, the United Nations war crimes prosecutor, has accused Serbia of protecting Bosnian Serb fugitives and said the former wartime leader Radovan Karadzic was living in the capital.
"I received just last week information from a credible source that Karadzic is now in Belgrade ... So Belgrade is now a safe haven for our fugitives," Mrs Del Ponte told reporters in Brussels.
Until now Karadzic has been thought to move between hideouts in Bosnia and his native Montenegro. Mrs Del Ponte said he appeared to have joined his former army chief, Ratko Mladic, in the city.
"So we have our most high- level people responsible for the crimes committed - Mladic and Karadzic - in Serbia," she said.
The two men have been indicted by the War Crimes Tribunal for genocide in the siege of Sarajevo and the massacre at Srebrenica, war crimes which claimed tens of thousands of lives in the 1992-1995 Bosnia war.
"Our fugitives are well protected in their own country because they are considered to be heroes. It is the best territory to hide," Mrs Del Ponte declared.
Serbian officials denied that the wanted men were in the country. Zoran Zivkovic said: "Carla Del Ponte has been claiming for three years that the fugitives are in Serbia, but she has not provided us with any evidence for that." He added: "If the suspects were in Serbia, they would have been arrested already. I do not see what else our government could do."
Mrs Del Ponte has often declared that Serbian authorities have turned a blind eye to alleged war criminals hiding on their territory. She believes about 15 war crimes suspects are on the run in Serbia.
Earlier this month, Mrs Del Ponte said Nato troops who raided the former headquarters of Karadzic in Pale in January had missed him by just two hours.
Two Bosnian Serb policemen who took part in that raid said yesterday they would sue Nato-led peacekeepers for using them as "human shields".
Nenad Sarenac and Rajko Mihajlovic said they wanted $1m each in compensation for putting their lives at risk when troops tried to enter Karadzic's abandoned house on 13 January.
Mr Sarenac said he and Mihajlovic were the only local policemen present during the raid on the house. "A masked man in plain clothes put hands on my shoulders and pushed me to open the courtyard gates and later to open the house door," he told Reuters. "He ordered me to bang on the door and shout: This is S-For [Stabilisation Force]."
Mr Mihajlovic said a uniformed S-For soldier did the same thing to him.
The two men said they would file a complaint at a local court and were ready to go as far as the European Court of Human Rights in Strasbourg.
A spokesman for S-For, Dave Sullivan denied the allegations. "I have taken a look at the videotape. S-For members were standing side by side with the [Bosnian Serb] authorities. Never once was anyone used as a human shield."Reuse content