Jacky Rowland in Belgrade: There is a strong danger of new fighting

Mr Holbrooke said five hours of talks in Belgrade with the Yugoslav President, Slobodan Milosevic, had been dominated by the recent upsurge in violence in the province.

On Monday, six Serb teenagers were shot dead by masked gunmen in Kosovo. Earlier, Yugoslav forces killed at least 30 ethnic-Albanians who were allegedly trying to smuggle arms into Kosovo.

"This meeting was planned before the events of yesterday," Mr Holbrooke said, "but of course they changed the agenda considerably and dominated our discussions."

Richard Holbrooke condemns the killings in Pec

He saved special condemnation for the killing of the six young Serbians: "All the incidents that threaten stability and lead to violence are lamentable...but the one that we find appalling beyond words is what appears to be a wanton attack on a group of primarily teenagers in the Panda bar in Pec."

The Serbian police are maintaining a strong presence in Kosovo

Mr Milosevic issued a separate statement, accusing the international community of failing to keep its promise to prevent attacks on Serbs by ethnic Albanian rebels.

"The terrorist gangs have not ceased attacking the army, the police, and inhabitants of Kosovo, " the statement said.

The BBC's correspondent in Belgrade, Jacky Rowland, says the task facing American negotiators now is how to narrow that gap before the situation on the ground in Kosovo deteriorates again.

As a next step, the American diplomat leading the self-rule discussions, Christopher Hill, is due back in Pristina on Thursday.

Informal truce

In October Mr Holbrooke brokered an informal truce in Kosovo and persuaded the Serbian president to withdraw most of his forces from the province under the threat of Nato airstrikes.

But many Serb forces remain and Mr Milosevic says he cannot withdraw completely for fear of handing over the province to ethnic Albanian rebels with dangerous consequences for the province's Serbian minority.

On Monday there were renewed clashes in the province when Serbian forces killed at least 30 ethnic Albanians and injured another 12 in the worst violence since the ceasefire was agreed.

Serbian border guards reportedly encountered an armed group wearing uniforms of the Kosovo Liberation Army, trying to enter the province illegally from Albania. According to the Serbian Ministry of Information, the shoot-out happened on Monday near the border outposts of Gorozup and Liken southwest of Pristina.

Large quantities of weapons and supplies were also reported to have been seized.