UN police forced to withdraw from Serb area of Mitrovica after storming courthouse.
US troops trained in riot control have been sent to the region to support the peacekeepers.
The northern part of the Mitrovica is dominated by ethnic Serbs, while south of the river the population is predominantly Kosovar Albanians.
Angry Serb demonstrators have been protesting against Kosovo’s declaration of independence from Serbia on February 17.
The United Nations in Kosovo has accused Serbian officials of encouraging the “orchestrated” clashes in which dozens of police officers were wounded.
“Some of the statements that Minister Samardzic [Serbia’s government minister for Kosovo] made when he was in Mitrovica yesterday were objectionable,” Larry Rossin, the second-in-charge of the UN mission in Kosovo, said on Tuesday.
“We’ve never had what we could consider a clear and unambiguous denunciation of this kind of violence from the ministers or indeed any other Belgrade government official that I can think of.”
But after the riots on Monday, Slobodan Samardzic, Serbia’s minister for Kosovo, told the Serb protesters that “we will protect you just like we protect the Serbs in Serbia”.
“We will reach the goal only if we are patient, smart and organised and if we believe in what we want to accomplish.”
‘Bled to death’
Larry Wilson, the senior UN police officer in Kosovo, said investigators were seeking those responsible for the death of Ihor Kynal, the Ukrainian policeman who died of injuries from a hand grenade.
“He basically bled to death,” Wilson said. “Because of the gunfire, it took us almost two and a half hours to evacuate him.”
One Serb demonstrator remains in a coma after being shot in the head.
Belgrade officials have accused the UN and Nato peacekeepers of using excessive force but the senior Nato commander in Kosovo said “appropriate force” was used and warned that the 16,000 strong force showed it would not give in to violence.
Condoleezza Rice, the US secretary of state, led appeals for calm, saying “all sides should refrain from violence”.
But Sergei Lavrov, Russia’s foreign minister, said at the same news conference that the problems had been caused by Kosovo Serbs’ rights being ignored in their own country.