Nato peacekeepers took control of a northern border crossing in Kosovo on Thursday morning, after the post was destroyed by ethnic Serbs.
The alliance said its personnel had been fired upon, when masked youths set fire to the Jarinje border post, throwing Molotov cocktails and then destroyed it with a bulldozer.
Agence France Presse said that heavily armed members of the Nato force in Kosovo could be seen on duty directing traffic across the border.
A Nato spokesman said the alliance had “enforced its presence” there.
Jarinje was one of two border posts stormed by Kosovan elite police units on Monday, who were seeking to enforce a ban on Serbian imports.
The units withdrew from the area, an enclave inhabited by about 60,000 ethnic Serbs, with Kosovan authorities announcing they had accomplished their mission, despite clashes with locals.
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A witness told AFP by telephone that the entire Jarinje post was “burned and covered with smoke".
The witness said about 25 police and border officers, including some from the European Union's EULEX mission, took refuge on the Serbian side of the border.
Mission spokesman Nicholas Hawton said there were no casualties.
Kosovo's Prime Minister Hashim Thaci said Belgrade was behind the attack. But Serbian President Boris Tadic blamed “hooligans” and “extremists”, and called for an end to the violence.
Meanwhile, the UN Security Council has called an urgent meeting in New York on Thursday, at the request of Serbia to discuss violence.