The latest ethnic violence between Kosovo's Albanian and Serb communities has killed at least 31 people and injured 500.
NATO has announced plans to send 1000 troops to bolster its 17,000 strong force.
The bloodshed is the worst in the Serbian province since it was put under UN administration at the end of the civil war in 1999.
The foreign minister of Serbia-Montenegro has told an emergency meeting of the UN Security Council swift action must be taken to prevent the slaughter.
"To the Serbs, the signal is that there is no life for them in the province and they should leave," Goran Svilanovic told the Security Council yesterday.
After the meeting he described the violence as an "organised attempt to ethnically cleanse Kosovo of Serbs".
Kosovo province, which has an ethnic Albanian majority, is formally part of Serbia-Montenegro but is under UN control.
"The worst case scenario is that many Serbs are going to flee and may not be back . . . we're going to see in several days how many of them have fled," Mr Svilanovic said.
As the violence continued, the US yesterday shut its embassy in Belgrade amid demonstrations in the Serbian capital following the latest killings.
The embassy said: "Tensions resulting from the violence in Kosovo have led to a continuing number of violent incidents in Belgrade and elsewhere in Serbia.
"The American Embassy and other foreign interests have been among the targets of unruly crowds."
In Belgrade yesterday riot police used teargas to disperse hundreds of students who tried to tear down the flag at the Albanian embassy, but allowed a larger, peaceful demonstration by thousands of flag-waving Serbs with banners that read, "We will not give up Kosovo".
Ethnic Albanians, frustrated with the long political stalemate in the UN-administered Serb province, also turned their fury on UN and NATO forces there with attacks on troops, police, vehicles and buildings.
UN international staff were also evacuated from Kosovo.