The violence, which broke out on Wednesday between Serbs and the majority ethnic Albanians, is the worst in the Serbian province since it was put under UN administration in 1999.
A source in Brussels on Thursday said up to 1000 soldiers would be deployed as back up for NATO's 17,000-strong KFOR peacekeeping force.
NATO's top military commander in Europe US General James Jones expressed concern about the flare-up.
"I call on the leaders of both sides of the conflict to take decisive action to control immediately their citizens and return to the rule of law," he said.
Kosovo is home to about 1.8 million ethnic Albanians and 80,000 Serbs. Several border towns in the far south of Serbia also have large Albanian communities.
World leaders fear that clashes of this kind could spin out of
control and destabilise the fragile Balkans region.
UN police deploy in Mitrovica to
prevent an escalation
A NATO spokeswoman said that three units - US and Italian from neighbouring Bosnia, and one provided by Britain -were ordered in. Britain said it was rushing in an extra 750 troops, adding they would be on the ground within four days.
The White House said on Thursday it was also sending a unit of US soldiers to Kosovo.
On the ground, commander of NATO-led troops in Kosovo (KFOR) German General Holger Kammerhoff in Pristina gave his troops the green light to use force to quash the violence.
Worship places targeted
NATO troops fired tear gas and rubber bullets at a crowd of ethnic Albanians who set a Serbian Orthodox church on fire in the flashpoint town of Mitrovica on Thursday.
French peacekeepers tried to prevent protestors from approaching the church in the southern side of the ethnically divided city, but people still managed to break into the building, said a Reuters journalist.
A Serb church was also set on fire in another Kosovo town on Thursday, in Obilic near the provincial capital Pristina.
In Brussels officials played down the sense of crisis. "It's perfectly normal. It's a routine thing. We don't go anywhere without having the plans in our pockets," said the spokeswoman.
But on the ground tensions are rising.
Angered by ethnic clashes in Kosovo that targeted their kin, Serb nationalists rampaged in Serbia, torching mosques and threatening Kosovo's ethnic Albanians with "slaughter and death."
The protests in Belgrade and other Serbian towns were reminiscent of similar nationalist outbursts at the start of the Balkan wars in the early 1990s.
A mosque in Belgrade is set on
fire by Serbian protestors
Then former President Slobodan Milosevic's propaganda deliberately incited hatred toward other ethnic groups then living in the former Yugoslavia.
The crisis in Kosovo began after a Serb teenager was shot and injured in the village of Caglavica, south of Pristina, on Monday and three Albanian children drowned after being chased into a river by Serbs in the ethnically divided northern town of Mitrovica on Tuesday.
Overnight in Belgrade, the capital of Serbia-Montenegro, demonstrators set a 17th century mosque - the city's sole place of worship for Muslims- on fire after clashing with police trying to guard the building.
The head of Serbia's Muslim community, Hamdija Jusufspahic, criticised the police for their "passive" protection of the mosque, which remained intact during the Serb-Muslim war in Bosnia but was gutted early on Thursday.
Demonstrators demanded that the government act to protect their Orthodox Christian kin in Kosovo from attacks by the province's predominantly Muslim ethnic Albanians.
In Nis, Serbia's second-largest city, 5000 people gathered on the main square, chanting "Slaughter, death to all" Kosovo Albanians, and "Let's all go to Kosovo!"
Crowds also protested in the city of Novi Sad, demolishing a local Islamic community headquarters and hurling stones at houses in a poor neighborhood believed to be inhabited by Muslims.
Serbian officials tried to calm the surging nationalist passions.
"Our churches won't be repaired by the destruction of mosques," said Serbia's senior official for Kosovo, Nebojsa Covic. He was referring to reports that numerous Serbian Orthodox churches were destroyed by ethnic Albanian rioters in Kosovo on Wednesday.
In Belgrade riot police guarding the US embassy used tear gas and stun grenades to disperse rock-hurling protesters, mostly drunk soccer fans, who overturned cars and garbage cans and destroyed buses. Serb nationalists consider the United States the biggest ally of Kosovo's ethnic Albanians.
Police said 24 officers were injured in overnight clashes in the capital, two of them seriously. At least 100 demonstrators also were hurt.