Nato has repeatedly failed to capture Mr Karadzic

Ms Del Ponte said she had received information from a credible source in the Serb capital last week.

She described Belgrade as "a haven for fugitives", adding that her office's relations with Serbia had been frozen.

Mr Karadzic, charged with genocide for his part in the Bosnian war, remains free eight years after the war ended.

We must say at this time co-operation is frozen. Belgrade is now a safe haven for fugitives Carla Del Ponte UN war crimes prosecutor

Ms Del Ponte said he had apparently joined his former army chief Ratko Mladic in the Serbian capital.

"So we have now both our most high level (people) responsible for the crimes committed, Mladic and Karadzic, in Serbia," she said.

Ms Del Ponte also criticised Serbian authorities for failing to co-operate with UN authorities.

"We must say at this time co-operation is frozen," she said.

"Belgrade is now a safe haven for fugitives."