PRISTINA, Serbia-Montenegro - NATO sent reinforcements to Kosovo today after 22 people were killed and hundreds injured in fighting between Serbs and ethnic Albanians in the worst violence since the province's 1998-99 war.

Arsonists today torched several Serb houses in Obilic, an ethnically mixed town west of the provincial capital of Pristina, forcing U.N. police and NATO troops to evacuate dozens of Serbs.

Bracing for more trouble, NATO mobilized extra units today, sending about 350 troops to the province, mostly from Bosnia and Italy, to beef up the 18,500 international peacekeepers now in Kosovo.

The breakdown in order illustrated the failure of U.N. and NATO efforts to snuff out ethnic hatreds and set the province on the path of reconciliation some five years after a NATO air campaign stopped a Serb crackdown on the independence-minded Kosovo Albanian majority.

NATO played down the prospects of renewed conflict, saying the alliance and the United Nations were committed to keeping the peace and quelling tensions.

"I don't believe there is a possibility of a war. We will do what is necessary to restore and uphold law and order," NATO spokesman Jamie Shea said today.

Serbs see the U.N.-run province as their ancient homeland. Ethnic Albanians want independence from Serbia-Montenegro. Hatreds between the two sides continue to boil over into violence.

All the deaths came in gunbattles, riots and street fighting on Wednesday. Evidence of the violence was still visible: Smoke billowed from Serb houses set ablaze in the ethnically mixed town of Kosovo Polje and burned out cars littered the streets of Pristina.

The clashes started in the ethnically divided city of Kosovska Mitrovica after ethnic Albanians blamed Serbs for the drowning of two of their children and began rampaging in revenge.

Melees broke out elsewhere in the U.N.-run province, including several enclaves where Serbs have eked out a sheltered existence since the end of the war.

NATO-led peacekeepers and Romanian police units moved in, firing tear gas, rubber bullets and stun grenades to stop ethnic Albanians from surging across a key bridge toward the Serb side of the city, where another crowd had gathered.

The new tally of casualties today was given by Angela Joseph, a spokeswoman for the U.N. police. Sixty-one police officers, including 40 members of the U.N. special police unit, were injured during the clashes, she said.

Separately, Lt. Col. Jim Moran, spokesman for the NATO-led peacekeepers, said that 17 peacekeepers were injured.