NATO troops secured a hostile strip of north Kosovo on Tuesday after Serb riots forced the pullout of U.N. personnel in the most serious challenge to the state since it split from Serbia last month.

Soldiers in infantry vehicles and armoured personnel carriers were positioned at key points in the flashpoint town of Mitrovica, where Serbs bitterly opposed to Kosovo's independence clashed with U.N. police and NATO peacekeepers on Monday.

Bridges over the Ibar river that divides the Serb north from the Albanian south were closed.

The U.N. mission that has run Kosovo since the 1998-99 war said the withdrawal of its police and civilian staff from the Serb stronghold of north Mitrovica was only temporary, but could not say when they would return.

Monday's clashes highlighted the risk of Kosovo's partition along ethnic lines and cast further doubt on the deployment in the north of a European Union police mission intended to take over much of the role of the U.N. administration in Kosovo.

"We will maintain our intention to deploy the mission throughout the territory of Kosovo," the EU's new Kosovo envoy, Pieter Feith, told a news conference.

The violence, sparked by a U.N. police operation to retake a U.N. court seized three days earlier by protesting Serbs, was the worst since Kosovo's 90-percent Albanian majority declared independence from Serbia on Feb 17.

GUNFIRE

NATO said its troops came under automatic gunfire as Serbs converged on the court following the dawn raid. Serb media reports said about 70 civilians were wounded, along with dozens of U.N. police and soldiers of the 16,000-strong NATO-led peacekeeping force.

The EU last month withdrew a small advance team from north Mitrovica for security reasons. A U.N. spokesman said U.N. staff would return "as soon as the security situation permits".

In Washington, State Department spokesman Tom Casey said: "The United States condemns the violence against U.N. and NATO personnel near the U.N. courthouse in Mitrovica, Kosovo.

"We urge all communities in Kosovo to remain calm and we call on the Serbian government to denounce these acts of violence and take affirmative steps to reduce tensions."

Backed by big-power ally Russia, Serbia has rejected Kosovo's secession and its recognition by the United States and a majority of the EU's 27 members.

Around 120,000 Serbs remain in Kosovo among 2 million ethnic Albanians. Almost half live in the north, adjacent to Serbia and in complete isolation from the capital Pristina. They reject the incoming EU mission as "occupiers".

Russia on Monday demanded restraint by NATO and Serbia said it was consulting Moscow on joint steps to protect Kosovo Serbs.

Serbia lost control over Kosovo in 1999, when NATO bombed to drive out Serb forces and halt the killing and ethnic cleansing of Albanians in a two-year Serb counter-insurgency war.

Belgrade is now strengthening a network of parallel structures in Serb areas of Kosovo, severing ties between Serbs and Albanians in all aspects of civic life.

"We have to be present here as a state to provide security for Kosovo Serbs," Serbia's Minister for Kosovo, Slobodan Samardzic, told Serbian state television late on Monday.

Addressing the crowd in Mitrovica, Samardzic said: "Our battle continues. Kosovo is part of Serbia."