Ethnic Serbs have attacked and set fire to a security post on Kosovo's northern border with Serbia.
They reject Kosovan independence and were protesting over attempts by police to take control of the border.
After the Kosovo police withdrew, it appeared a deal had been struck and there was a lull, but the Serbs attacked again in the early evening.
Serbia's President, Boris Tadic, has urged an immediate end to the violence, calling the protesters "hooligans".
The latest outburst prompted Nato peacekeepers to deploy at the Jarinje crossing, and at another crossing which they feared could also be targeted.
There are as yet no reports of injuries, but these are the worst clashes in several months, according to the BBC's Mark Lowen in Belgrade, Serbia.
One of the Kosovan police officers was killed and several others wounded in the initial operation, when local ethnic Serbs shot at them.
Appeal for calm
Previously the area was principally under the authority of the European Union.
It is Serb-dominated, however, and the police operation was motivated in part by dissatisfaction on the part of the Kosovan authorities in Pristina with the performance of ethnic Serb police officers who manned the border crossing.
Pristina also wanted to enforce a ban on Serbian goods from entering the territory, after an equivalent import ban from Serbia.
The police operation was criticised by the EU and United States as provocative.
Nicholas Hawton, spokesman for the EU police mission in Kosovo, condemned the violence and appealed for people to remain calm.
But the Kosovan Prime Minister has defended the police actions as a legitimate attempt to assert control over the north, an area which has never accepted the mandate of the Kosovan government.
The same crossing was attacked in 2008, after Kosovo unilaterally declared independence from Serbia, a move still rejected by the Serb community.
Serbia refused to recognise the formal secession declared by the ethnic Albanian majority in 2008, and maintains close ties with the region north of Mitrovica, where ethnic Serbs have their main enclave.
On Thursday an urgent UN meeting will discuss the crisis.
The government in Serbia has warned that a tense night lies ahead in northern Kosovo - still one of the most unstable corners of Europe.