Chief UN prosecutor Carla Del Ponte - giving the first concrete figure on deaths in Kosovo - told the UN Security Council that so far, only about a third of the 529 grave sites had been examined.

Altogether, more than 11,000 deaths have been reported to her prosecutors.

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.

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

In her briefing, Carla 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."

Nato reports continuing ethnic violence

In a separate development, NATO has said that nearly 400 people had 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.