Ethnic Albanians torched Serb homes and churches Thursday as Kosovo (search) convulsed in a second day of rioting. The worst violence since the province's war ended in 1999 has killed at least 31 people and injured hundreds.

Serbian nationalists set mosques elsewhere on fire and threatened to retaliate with "slaughter and death." NATO (search) sent reinforcements to quell tensions in the U.N.-run province and ease the threat of renewed conflict in the volatile Balkans.

Stung by the lawlessness that have left Serb enclaves here in ruins, peacekeepers promised to respond to provocations with a level of force not used here in the past.

Some peacekeepers were already carrying out the orders, shooting and wounding protesters who used violence in clashes Thursday, said Col. Horst Pieper, the chief NATO spokesman in Kosovo. The number of injured peacekeepers rose to 51 since clashes began Wednesday.

"The soldiers ... will not tolerate those who seek to cause harm," said U.S. Brig. Gen. Rick Erlandson said in a statement. "My soldiers will immediately and forcefully stop anyone who violates the rule of law."

The clashes, which began Wednesday when ethnic Albanians blamed Serbs for the drownings of two children, have killed at least 31 people and injured hundreds, officials said.

The bloodshed underscored the bitter divisions that have polarized Kosovo's mostly Muslim ethnic Albanians, who want independence from Serbia, and Orthodox Christian Serbs, a minority in Kosovo who consider the province their ancient homeland.

The violence, which spilled beyond Kosovo's borders into the Serbian heartland, also dealt the Bush administration a potential setback in efforts to reduce the number of peacekeepers in the Balkans and redeploy them to Iraq, Afghanistan and other hotspots. About 2,000 Americans now serve with the force, down from 5,000 after the war, and the entire force has shrunk from 50,000 to 18,500.

"The international community's drive to reduce [NATO] forces and the U.N. police for cost reasons and because of Iraq has turned out to be an error," warned Winfried Nachtwei, a German lawmaker who visited Kosovo this week.

The White House called for an end to violence in Kosovo and said President Bush met with his national security team to monitor the situation. The U.S. Embassy in Belgrade closed temporarily to the public as a precaution.

"We continue to call on all groups to end the violence and refrain from violence," White House spokesman Scott McClellan said.

Serbia-Montenegro's military raised the combat readiness of some units to their highest level, and Kosovo's ethnic Albanian prime minister, Bajram Rexhepi, warned that the situation was not under control. The U.S. Embassy in Belgrade closed to the public temporarily as a precaution.

But the United Nations and NATO, which was bolstering its 18,500-member peacekeeping force with 1,100 more troops, played down the risk of fresh conflict in Kosovo or elsewhere in the region.

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

At the U.N. Security Council, the foreign minister of Serbia-Montenegro denounced attacks on Serbs in Kosovo as a renewal of ethnic cleansing in the Balkans.

"What we see happening is an organized attempt to ethnically cleanse Kosovo of Serbs," Goran Svilanovic told The Associated Press after the council meeting.

In the flashpoint city of Kosovska Mitrovica, U.N. civilian staff in the ethnic Albanian-dominated south were relocated to a French military base late Thursday, a U.N. official said on condition of anonymity.

About 100 Serbs were evacuated Thursday from the center of Pristina and other communities by police and NATO-led peacekeepers, officials said. Some of the apartments and cars they left behind were burned by arsonists.

A mob surrounded and set an Orthodox church on fire in a suburb of Kosovo's capital, Pristina. Police fired tear gas and rubber bullets to push people back. Gunshots were heard throughout the capital and military helicopters hovered over the crowds.

Also Thursday, an explosion rocked the center of the town of Lipljan. NATO-led peacekeepers determined the blast was caused by about 20 grenades stashed behind the municipal offices, said Angela Joseph, a U.N. police spokeswoman.

An ethnic Albanian mob clashed Thursday with peacekeepers and U.N. police on a road leading to the Serb enclave of Caglavica, the scene of street fighting a day earlier. Peacekeepers fired tear gas and rubber bullets in an attempt to disperse the crowd. It was unclear whether anyone was hurt.

In Belgrade, the capital of Serbia-Montenegro, demonstrators set the city's 17th century mosque on fire after clashing with police. A mob also torched a mosque in Nis, Serbia's second-largest city.

Branko Crvenkovski, the prime minister of neighboring Macedonia -- still tense after ethnic Albanian militants waged a six-month insurgency in 2001 -- said he saw no danger the violence could spread there.

Unwilling to take any chances, the NATO alliance deployed 350 U.S. and Italian soldiers from Bosnia to Kosovo, and Britain said it would soon send 750 troops.

Arsonists on Thursday torched several Serb houses in Obilic, an ethnically mixed town west of Pristina, forcing U.N. police and NATO troops to evacuate dozens of Serbs. There were unconfirmed reports that the town's Serbian Orthodox church also was ablaze.

Harri Holkeri, Kosovo's top U.N. official, appealed for an end to the violence, warning that it undermined the international community's efforts to reconcile the two sides after the war. The conflict killed about 10,000 people, mostly ethnic Albanians.

It ended in 1999 after NATO airstrikes halted former Yugoslav President Slobodan Milosevic's (search) brutal crackdown on ethnic Albanian militants seeking independence. Ethnic Albanians since have mounted periodic revenge attacks on Serbs.

Kosovo is U.N.-administered but remains part of Serbia, with its final status to be decided by the United Nations.