Nato troops fired tear gas and gunfire was heard in north Kosovo late on Wednesday as hundreds of Serbs poured into the streets to defend barricades erected against the country's ethnic Albanian authorities.
Sirens called Serbs out to respond as Nato soldiers moved to dismantle one of more than a dozen roadblocks erected in July against an operation by the Kosovo government to post border police in the mainly Serb north, a Reuters witness said.
Kosovo, where 90% of the 1.7 million people are ethnic Albanians, declared independence from Serbia in 2008. But Serbs in a small slice of the north bordering Serbia reject the secession, and the West has struggled to tackle the country's de facto ethnic partition.
Serbia itself is under pressure to have the barricades removed and make progress in ties with Kosovo if it is to clinch candidate status for future membership of the European Union when the bloc meets on Dec 9.
A Reuters witness said crowds of Serbs had forced back Portuguese and Hungarian NATO troops who had approached on foot near the northern town of Zvecan.
Serbs tipped truckloads of concrete blocks at the site, reinforcing the barricade as the troops, part of Nato's 6,250-strong Kosovo Force (KFOR), pulled back.
Gunfire rang out in the main town of Mitrovica, but it was unclear who was shooting. There were no reports of casualties in the violence.
The EU deployed a police and justice mission to Kosovo after its declaration of independence -- recognised by 22 of the EU's 27 members -- but the mission is unable to operate freely in the north due to Serb resistance.
Serbia lost control over Kosovo in 1999, when NATO bombed for 78 days to halt the killing and expulsion of ethnic Albanians in a two-year Serb counter-insurgency war under late strongman Slobodan Milosevic.
More than 80 countries have recognised the new country, the last to emerge from the remains of Yugoslavia.