Pristina, Serbia-Montenegro ? Ethnic Albanians torched Serb homes and churches Thursday as Kosovo 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 sent reinforcements to quell tensions in the U.N.-run province and ease the threat of renewed conflict.

Stung by the lawlessness that has 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,” 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 began Wednesday when ethnic Albanians blamed Serbs for the drownings of two children.

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.