PRISTINA, Yugoslavia (AP) _ Unknown attackers fired Sunday on a joint American-Russian patrol trying to seal the boundary between Kosovo and part of southern Serbia where ethnic Albanian rebels have been challenging Yugoslav forces, the U.S. Army said.

There were no casualties in the attack, the first reported against NATO-led peacekeepers since ethnic Albanian rebels escalated cross-border raids in November. The joint patrol returned fire but it was unclear whether the attackers suffered casualties, U.S. Army spokeswoman Capt. Alayne Cramer said in a statement.

The attack occurred one day after a violent incident in northwestern Kosovo, in which Serbs angry over the arrest of a motorist set fire to a police station, stoned vehicles and briefly took seven Belgian soldiers hostage.

Two Serbs died in the melee in the town of Leposavic and one was wounded, raising tensions in this troubled province.