STRPCE, Yugoslavia (Reuters) - International peacekeepers in Kosovo came under attack on Tuesday after a busload of Serbs they were escorting ran into an ambush that killed one passenger and injured two, the peacekeepers said.

``One Serbian was killed today and two wounded when a KFOR-escorted convoy was fired upon by an unknown assailant near Grlica,'' said a KFOR spokesman in Kosovo's capital Pristina.

``The bus continued on to Strpce and took the wounded to the medical center there,'' he added.

Immediately after the ambush, angry Serbs gathered at the local headquarters of KFOR and the United Nations (news - web sites) police in Strpce, southern Kosovo, and threw petrol bombs and rocks, badly damaging seven KFOR and U.N. vehicles, an eyewitness said.

Peacekeepers fired shots in the air but the violence continued, the witness said, adding that he had seen an UNMIK (United Nations Mission in Kosovo) policeman and several civilians injured.

Major Jim Marshal, a U.S. spokesman for the NATO (news - web sites)-led KFOR peacekeeping force, told Reuters on Tuesday evening the situation had calmed but protests were continuing.

``Right now in Strpce demonstrations are going on, there are between 500 and 700 demonstrators. They have burned several vehicles,'' he said by telephone.

Kfor Sends Reinforcements

``At the moment there are some 200 KFOR peacekeepers at the scene and they are attempting to stabilize the crowd without further violence. Currently there are KFOR units from Ukraine, Poland, Italy, Greece, United Arab Emirates and us,'' he said.

UNMIK spokeswoman Susan Manuel said in Pristina all U.N. personnel had been evacuated safely from their buildings in Strpce, where the demonstrators had rung church bells to call others to join them.

She had no word on the reported injury to its policeman.

Dragan Buduric, leader of the local Serb National Council grouping, said some 40 people had been on the bus which was traveling from the northern town of Podujevo to Brezovica, near Strpce, when the attack happened at about 4 p.m.

He said a 46-year old Serb man was killed in the ambush, the most serious attack on a Serb convoy since a U.N. refugee agency bus was hit by a rocket-propelled grenade in northern Kosovo a year ago.

Following that attack, in which two Serbs were killed and several injured, Serbs went on the rampage in the nearby town of Mitrovica, killing eight Albanians and forcing others to flee their homes.

Since the end of NATO's 1999 bombing of Yugoslavia, Kosovo's Serb minority has been the frequent target of violence from members of the ethnic Albanian majority who were repressed for years by the Serbian state under Slobodan Milosevic (news - web sites).

In separate incident not far from Strpce, KFOR said a Serbian man had been shot at in a field by an unknown gunman.

``The man, who was tending his livestock at the time, was shot three times in the left leg by his assailant, and received medical assistance from KFOR soldiers on the scene. He was taken by helicopter to the Camp Bondsteel hospital where he is in stable condition,'' it said.

In northern Kosovo, three Serbs were wounded when their tractor hit a land mine near the village of Suvo Grlo, a U.N. police spokesman said.