Flights in and out were suspended and internal boundaries with Serbia were closed after violence between Kosovo's Albanian and Serb communities killed at least 22 people and injured some 500 others, a UN official said on Thursday.
"We have 22 victims and some 500 injured, out of which 61 are police and 11 are NATO-led peacekeepers," UN police spokesperson Angela Joseph told the news agency AFP.
NATO said on Thursday it was sending reinforcements to Kosovo from Bosnia to help quell the violence, with a company of 100 to 150 US troops already on its way and two others on standby.
"One company from SFOR (NATO's Stabilisation Force in Bosnia) and two others are on standby," a NATO spokesman said. "They will be put into theatre and put at the disposition of the commander wherever necessary."
Spokesman for SFOR Dave Sulliven said plans to send troops to Kosovo were underway to help 17,000 NATO troops already there with KFOR.
The violence was triggered on Tuesday in a northern town by a report that three ethnic Albanian children were pushed into a swift-flowing river by a group of Serbs and drowned.
Peacekeeping troops from more than a dozen nations patrolled key areas, in some cases next to gutted Serb buildings.
It was the scale of the violence rather than the death toll which signalled a crisis.
In a severe blow to international hopes of calm ahead of talks this year or next on Kosovo's future status, the outburst of pent-up hatred in more than a dozen locations suggested that reconciliation of the two communities was still a long way off.
Clashes from north to south
Clashes were reported from Mitrovica in the north to Urosevac in the south and Pec in the west, with UN police and troops injured in several places, at least three gravely.
"The idea of an multi-ethnic Kosovo, if it ever existed, was definitively buried on Wednesday and now belongs to history"
Serbian human rights minister
The violence triggered angry protests in Serbia's three main cities, where demonstrators stoned and burned mosques and Islamic buildings.
Kosovo has been under the control of the United Nations since NATO bombing drove Serb forces out in mid-1999, halting Serb repression of Muslim Albanian civilians but also granting victory to Albanian separatist guerrillas.
Fuelling fears that impatient Albanians might turn on their NATO and UN saviours if independence is delayed, Serb mobs clashed with peacekeepers and police across the province.
United Nations Kosovo police veteran Derek Chappell said he believed the violence was coordinated. Kosovo Serb politician Momcilo Trajkovic said: "We are back in 1999," when Albanians took revenge on fleeing Serbs as NATO forces moved in.
Wednesday's violence began when Albanians massed in Mitrovica to vent their rage at Tuesday's drownings.
A survivor had said the boys were hounded into a river by Serbs, who were exacting revenge for a teenager wounded in a drive-by shooting.
More than a thousand Serbs took
part in a protest rally in Belgrade
Shooting broke out and grenades were thrown as police and troops fired tear gas and rubber bullets to stop Albanians storming the Serb half of the town.
Hundreds of angry Albanians surrounded a Serb enclave in Pristina, setting UN Jeeps ablaze and stoning police who fired rubber bullets.
US troops were evacuating Serbs whose apartments were under attack. US troops in a convoy of 30 armoured cars evacuated Serbs from Caglavica.
Serbia's minister of human rights said on Thursday that hopes of a multi-ethnic state in Kosovo have been dashed.
"The idea of an multi-ethnic Kosovo, if it ever existed, was definitively buried on Wednesday and now belongs to history," the official, Rasim Ljajic, was quoted as saying by Tanjug news agency.
"We have 22 victims and some 500 injured, out of which 61 are police and 11 are NATO-led peacekeepers"
UN police spokesperson
Ljajic, who is a Muslim like the Albanian majority in Kosovo, said the violence "clearly showed that the Albanians want Kosovo to be not only independent but also ethnically pure".
In Belgrade, Serb demonstrators broke through a police cordon and set fire to a mosque.
Witnesses said demonstrators also smashed windows of the US embassy. There were protests in the northern city of Novi Sad and a mosque was burned in the southern city of Nis.
International community blamed
But he laid most of the blame for the events on the international community "which has the most power in Kosovo" and called on it to change its policies.
The 12,000-strong SFOR has been deployed in Bosnia since the end of the country's 1992-95 war. NATO forced the Serbian army out of Bosnia so as to end the oppression of the Albanian majority.