Kosovo Serbs have set fire to two border crossings to protest against Kosovo's declaration of independence.
The attacks took place at the northern Jarinje and Banja crossings, manned by United Nations and Kosovo police.
In response, Nato-led peacekeepers were deployed at the crossings. There have been no reports of any injuries so far.
This is the most serious incident since Kosovo declared independence from Serbia on Sunday, the BBC's Nick Thorpe in Kosovo says.
Belgrade has said Kosovo's declaration violates international law.
See a map of Kosovo's ethnic breakdown
The US, Britain, France, Germany and Italy have all recognised the new state, but others have not.
Russia has warned that the move endangers international stability, while China has expressed its deep concern.
The UN Security Council is divided over how to respond to Kosovo's move, and it has failed to agree on any action.
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