Nato has deployed peacekeepers in the north of Kosovo after a border post with Serbia was burnt down and a Kosovo policeman killed.
Serbian nationalists in Kosovo attacked the post on Wednesday after Kosovo's government sent police to enforce a ban on imports from Serbia.
Border posts in the area, which rejects Kosovan independence from Serbia, had been staffed by ethnic Serb police.
Kosovo's government suspected them of turning a blind eye to banned imports.
Nato officials said only small vehicles were being allowed through two border posts at the centre of the dispute, and that they were being checked for weapons.
Serbia has also reinforced police on its side of the border, to prevent "extremists from Serbia going to Kosovo", Serbian police chief Milorad Veljovic told Reuters news agency.
Wednesday's violence was the worst in several months, according to the BBC's Mark Lowen in Belgrade, Serbia.
It began when a group of about 200 Serbian nationalists approached the Jarinje crossing, throwing firebombs.
They forced the Kosovan police and customs officers, and the EU police assisting them, to flee across the border into Serbia.
The post was burnt down and bulldozed.
One of the Kosovan police officers was killed and several others wounded when they were shot at.
The move by Kosovo's Prime Minister Hashim Thaci to reassert Pristina's control over the region has been criticised by the EU and the US as provocative.
The region has not accepted the mandate of the Pristina government since Kosovo's ethnic Albanian majority declared independence from Serbia in 2008.
Serbia itself has also refused to recognise its former territory's independence.