JARINJE, Kosovo - A mob of about 200 Serbs hurled firebombs and a customs post was set ablaze on Kosovo's northernmost border with Serbia on Wednesday, hours after Kosovo's special police withdrew from the area.

There were no immediate reports of injuries. The attack -- witnessed by an Associated Press reporter -- comes as Kosovo police said an operation to take control of two disputed border posts in the Serb-run north had ended.

Prime Minister Hashim Thaci ordered the police action on Monday. One policeman died during the operation.

There were reports of an exchange of fire between armed Serb civilians and NATO peacekeepers, but the claims could not be independently verified.

The European Union has condemned the police action and NATO peacekeepers have been deployed to try and ease tensions.

The violence is a serious setback for NATO-led efforts to broker a deal that would leave Kosovo officials in charge of the border crossings.

NATO's Kosovo commander, Maj. Gen. Erhard Buehler met Serbia's top officials for Kosovo in search of a solution, but a breakthrough did not appear imminent.