Serbian Deputy Information Minister Miodrag Popovic said: "I am very sceptical about the figure and about the place where the figure came from."

He was reacting to a statement by chief UN prosecutor Carla del Ponte to the UN Security Council in which she outlined the tribunal's work in Kosovo so far.

Giving the first concrete figure on deaths in Kosovo, Ms del Ponte told the Council that only about a third of the 529 grave sites had so far been examined, where 4,266 bodies were reported to have been buried. The teams have so far exhumed 2,108 bodies.

To date, more than 11,000 deaths have been reported to her prosecutors, but not verified.

Previous estimates suggested that the Serb forces may have left behind the remains of up to 10,000 ethnic Albanians when they withdrew from Kosovo five months ago.

Mr Popovic did not give his own estimate of the number of dead, but acknowledged that not all those killed in Kosovo were either "terrorists" or died due to Nato bombs, as the government had said at the time.

He said Serb judges could not verify the figures because they could not inspect the sites.

"Our judiciary doesn't have any access to the alleged sites, so how can we believe in what they say. They act as judge and jury at the same time," Mr Popovic said.

The UN investigators are gathering evidence relating to charges of crimes against humanity against Yugoslav President Slobodan Milosevic and four associates.

In her briefing, Ms del Ponte said that it was crucial to determine "the magnitude of the disaster" soon.

Only one-third of the mass grave sites have been investigated

She said that there was a continuing risk that some of the remaining mass grave sites might be disturbed before her investigators were able to examine them.

"There are also a significant number of sites where the precise number of bodies cannot be counted," she said.

"In these places, steps were taken to hide the evidence. Many bodies have been burnt, but at those sites, the forensic evidence is nevertheless consistent with the accounts given by witnesses of the crimes."

In a separate development, Nato has said that nearly 400 people have been killed in Kosovo since its troop took control of the region.

Roughly half of these were ethnic Serbs, although Serbs make up a small percentage of the current population of Kosovo.