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.

One of the Kosovan police officers was killed and several others wounded in the initial operation, when local ethnic Serbs shot at them.