According to Reuters news agency, NATO officials announced Thursday that an additional 100 to 150 U.S. soldiers were already on their way to Kosovo. At least 18 people were killed and more than 270 injured Wednesday as the UN-administered Serbian province of Kosovo became embroiled in some of the worst outpourings of violence between ethnic Albanians and Serbs in three years.

The conflict broke out early Wednesday morning amid reports that Serbs in a village near Kosovska Mitrovica set a dog on a group of ethnic Albanian boys, forcing them to flee into an icy river where they drowned. After authorities recovered the bodies, Kosovo-Albanians and Serbs faced off against each other near a bridge over the Ibar River that divides the city -- long a flashpoint of ethnic tensions -- and traded insults, threw rocks and charged at each other several times before gunfire broke out.

NATO-led peacekeepers and Romanian special police units in riot gear moved in, firing tear gas, rubber bullets and stun grenades to stop ethnic Albanians from rushing across the bridge toward the Serb side of the city.

Official reports said six ethnic Albanians and two Serbs were killed in the clash in Kosovska Mitrovica. Seven police officers and 11 French KFOR soldiers were injured.

Flashpoint of ethnic tensions

With emotions running high in the volatile region, the conflict quickly fanned out, spilling over into other towns where Serb homes, churches and cars were set on fire as ethnic Albanians sought revenge.

In the central Kosovo town of Lipljan, four Serbs were killed late Wednesday and three others died in the eastern town of Gnjilane as NATO and UN troops struggled to stop the violence. But in the course of the day, events became too much for the international peacekeepers to contain as ethnic Albanians rushed KFOR checkpoints established to protect Serb enclaves, and UN vehicles were reportedly torched in the capital Pristina, according to Beta news agency. Several medieval Serb Orthodox churches and monasteries were also allegedly set ablaze in central and east Kosovo.

Serbs demand retaliation

Meanwhile in Belgrade, the capital of Serbia-Montenegro, demonstrators set the city's 17th century mosque on fire after clashing with police trying to guard the building. The angry mob demanded that the Serbian government act to protect their Orthodox Christian kin from attacks by the primarily Muslim Kosovo-Albanians. A second mosque was torched in the southern Serbian town of Nis.

News reports from Belgrade said more than 10,000 people rallied outside the Serbian government building blocking main streets in the city center. Chanting, "We'll go to Kosovo" and "Serbia rise up," they demanded their leadership take action.

Prime Minister Vojislav Kostunica of Serbia, to which Kosovo belongs, appealed to his countrymen for an end to the violence and condemned what he called Albanian terrorism. "The events ... demonstrate the true nature of Albanian separatism, its violent and terrorist character," he said after an emergency cabinet meeting.

NATO and UN respond

In Brussels, NATO Secretary General Jaap de Hoop Scheffer called on both Serbs and ethnic Albanians to remain calm. "I urge all ethnic communities in Kosovska Mitrovica and in the rest of Kosovo to avoid further escalation, to act with calm and to refrain from demonstrations and roadblocks," he said in a statement.

UN Secretary General Kofi Annan urged all parties to halt the violence, "which jeopardizes the stability of Kosovo and the security of all its people."