PRISTINA, Yugoslavia (AP) _ Serb forces unleashed a new assault on Kosovo's Albanian rebels Friday, reportedly killing at least 15 separatists in a crackdown on three villages. Elsewhere, two cease-fire monitors were shot in the first attack to wound an international observer.

The violence in southern and western Kosovo dashed hopes that tensions in the troubled province would ease following the release of eight captive Yugoslav soldiers by the rebel Kosovo Liberation Army.

The attack on unarmed monitors is sure to raise new questions about their role in Kosovo, where up to 2,000 people have been killed since Yugoslav President Slobodan Milosevic launched an offensive against separatists last February.

Kosovo is a province in Serbia, the main republic in Yugoslavia. At least 90 percent of Kosovo's people are ethnic Albanian, and most favor independence.

International officials could not confirm the Serb report that 15 guerrillas had been killed. But they protested the crackdown by Yugoslav forces, who used tanks and heavy weapons in a display of military might that sent villagers fleeing into the nearby hills.

Britain, France and the United States all voiced concern and anger that a Briton and a Serb translator for the Kosovo Verification Mission had been shot _ with U.S. State Department spokesman James P. Rubin calling the situation ``unacceptable.''

``Both parties, the Kosovo Albanian side and the Serbian government, are responsible for the security of these monitors,'' Rubin said in Washington.

The Serbs' fierce assault around Stimlje, 15 miles south of Kosovo's capital of Pristina, began Friday morning. International monitors said government forces shelled and fired automatic weapons on three villages, and reporters who reached the area saw three tanks perched on hills overlooking the villages, with at least one firing down.

The Serb Media Center, whose reports of casualties have proven reliable, said at least 15 KLA fighters were killed in the village of Racak and nearby Petrovo. Citing police sources, it said police confiscated large quantities of arms and drove off the rebels.

Serb forces claimed to be seeking out rebels who killed a policeman and attacked a patrol in the area last weekend, according to the Serb center. Authorities said the same group was responsible for killing seven pro-government ethnic Albanians in southwestern Kosovo.

If confirmed, the death toll would be the second-highest in a single clash since a U.S-brokered Kosovo cease-fire in October. Thirty-six rebels were reported killed last month by Yugoslav army border guards who intercepted them as they were trying to smuggle in arms from neighboring Albania.

The ethnic Albanians' Kosovo Information Center reported an initial death toll of seven from the clashes. International monitors said Serb police blocked them as they tried to reach the area.

The Organization for Security and Cooperation in Europe, which runs the monitoring mission, said their employees were in a convoy of three OSCE vehicles near Decani in western Kosovo when unidentified gunmen ``deliberately targeted'' them.

The two victims were taken to a Pristina hospital, and the Serb translator was later released. The Briton was taken to Skopje in neighboring Macedonia for further treatment after being shot in the shoulder. OSCE spokesman Mons Nyberg declined to release further details.

British Foreign Secretary Robin Cook said he was ``shocked and concerned'' to hear of the shootings. ``The work of the KVM benefits both sides in Kosovo, and I absolutely condemn any violence directed towards them,'' he said.

In Sarajevo, Bosnia, the supreme NATO military commander, Gen. Wesley Clark, expressed concern that the violence in Kosovo was affecting chances for peace.

``The longer the use of force goes on inside Kosovo, the more intractable the problem is likely to become,'' Clark said.