NATO Secretary-General Jaap de Hoop Scheffer, speaking at a news conference in Prague, said NATO's KFOR peacekeepers are determined to bring the clashes to a halt.
"KFOR is doing everything it can, including bringing in reserves, to see that the violence stops," de Hoop Scheffer said.
NATO spokesman James Appathurai told RFE/RL that NATO has called upon all sides to take concrete action to bring violence in the province under control.
"[NATO ambassadors] strongly deplored the violence that has flared-up there, which is causing casualties on all sides. It's causing extensive damage to property, including to churches and to mosques, and it's undermining the very important political dialogue between Pristina and Belgrade. They called upon all leaders in the area to take concrete action to control the violence and to bring an end to it. The [NATO] secretary-general [Jaap de Hoop Scheffer] has called the prime minister of Kosovo, he has also called the president and the foreign minister of Serbia and Montenegro to [relay] that message, he has been in touch with Mr. Holkeri, the UN's high representative in Kosovo to consult with him," Appathurai said.
Appathurai also said NATO decided to send reinforcements from Bosnia to Kosovo and reiterated that the alliance was in control of the situation.
"The allies also agreed to reinforce KFOR, so troops are coming in from next door in Bosnia, from SFOR, and the United Kingdom is also sending a battalion in, hopefully within the next few days to reinforce KFOR. The alliance is convinced that KFOR is doing its job, that it has the resources necessary, and this extra deployment is a demonstration of NATO's will and its capability to do whatever is necessary to ensure that KFOR can carry out its mission," Appathurai said.
In London, British officials announced that they will send some 750 troops to Kosovo in the next four days.
Protests, meanwhile, resumed today in Kosovo's ethnically divided town of Mitrovica, with international security forces standing on a bridge between the majority Kosovo Albanians and minority Serbs.
However, reports say ethnic Albanians set a Serbian Orthodox church on fire today in the city.
Ethnic Albanians also gathered in the province's capital, Pristina, and in Pec, in the southwest.
Arsonists today torched several Serbian houses in Obilic, an ethnically mixed town west of the provincial capital of Pristina, forcing UN police and NATO troops to evacuate dozens of Serbs.
In Serbia proper, nationalists set mosques on fire and threatened Kosovo's ethnic Albanians.
In Pristina, the commander of KFOR, General Holger Kammerhoff, gave his troops the green light to use force to quash the violence. U.S. troops are now patrolling the road linking Pristina and Mitrovica, and NATO has sent 150 U.S. troops and 80 Italian carabinieri.
Clashes initially flared yesterday in Mitrovica after claims that Serbs had chased three ethnic Albanian boys into a river. Two of the boys drowned; one remains missing.
Fighting continued overnight, with ethnic Albanians in locations throughout Kosovo burning the homes and property of minority Serbs, who were being evacuated by NATO-led forces.
A UN police spokesman today put the death toll at at least 22. About 500 people were injured, including 40 members of the UN special police and 11 peacekeepers.
In Serbia, the Interior Ministry put paramilitary police on the internal boundary with Kosovo on high alert.
Serbian Prime Minister Vojislav Kostunica today said the violence in Kosovo was "planned and organized" by ethnic Albanians bent on driving remaining Serbs from the province.
He urged the UN Security Council to introduce "extraordinary measures" to deter what he called an attempt at ethnic cleansing.
Kostunica also slammed NATO and the United Nations for "inadequate protective measures for Serbs in Kosovo."
In Moscow, a Russian Foreign Ministry spokesperson Alexander Yakovenko called for an emergency session of the United Nations Security Council to discuss the wave of violence.
The Russian call followed a similar one made last night by the foreign minister of Serbia and Montenegro, Goran Svilanovic.
Kosovo has been under the control of the United Nations since NATO bombing drove Serbian forces out in mid-1999, halting Serbian repression of ethnic-Albanian civilians.
Critics have said that recurring violence is dealing a severe blow to international hopes of calm before talks on Kosovo's future status, and that it threatens to throw the province back to where it was in 1999.
But analyst Verena Knaus, of the Pristina-based European Stability Initiative, a research and policy institute, told RFE/RL that despite the current wave of violence, progress has been made toward stabilizing Kosovo.
"There has been a lot of substantive progress made in the last years since 1999. There has been a noticeable increase in the freedom of movement of the Serbian community, there have been increasing cases of economic cooperation. There has been a political process that has been increasingly established, functioning institutions, there have been the first meetings of working groups. So on many levels there has been a lot of progress and people in Kosovo, I think, generally felt it in their daily lives. I mean, even the electricity situation has improved in the last few months," Knaus said.
All the same, Knaus said that the current wave of violence is a wake-up call that shows that much more remains to be done.
"What is clear and what has been shown yesterday is that this [tension] can flare very quickly, that there are underlying tensions and that we need to really focus on the real issues at stake -- on the economic problems, on the social problems. And we need to address them head-on and try to build a local consensus around them, try to find local solutions for these problems, try to find a compromise which both sides can accept. And in many ways, if we look at these incidents now, we should see them as a wake-up call to actually develop a more pro-active policy, especially for Mitrovica," Knaus said.
(RFE/RL's Bruce Jacobs contributed to this report.)