"The commitment of NATO KFOR to the implementation of its mandate is unwavering, as demonstrated by the recent additional deployment of 500 troops," alliance spokeswoman Oana Lungescu said.

"We once again remind all parties of their obligations to KFOR... including respect for the Administrative Boundary Line, procedures for the Kosovo Police and the 2013 Agreement on the deployment of the Kosovo Security Forces to the north," she tweeted.

NATO's KFOR has swelled to more than 4,000 troops after a battalion of 500 reinforcements from Turkey was sent to Kosovo two weeks ago, following clashes with ethnic Serb protesters in late May that wounded 30 of the peacekeepers.

An extra NATO battalion is on alert to deploy to Kosovo if required.

Serbia is holding three Kosovo police officers who were detained on Wednesday.

Kosovo's government has called it a kidnapping and, as a "security measure", banned all Serbian trucks from entering its territory.

The United States has strongly called for Serbia to release the police officers. Washington and Brussels are both urging a de-escalation of tensions.

EU foreign policy chief Josep Borrell has called on Serbia President Aleksandar Vucic and Kosovo's premier Albin Kurti to meet in Brussels in the next few days to agree on ways to ease the situation.

Kosovo declared independence from Serbia in 2008. Its ethnic Albanian majority rose up in 1999, sparking a NATO military intervention against Serbia's forces.

But Belgrade and its allies Beijing and Moscow refuse to recognise its independence, effectively preventing Kosovo from having a seat at the United Nations.

Serbs in Kosovo remain largely loyal to Belgrade, especially in the north, where they make up a majority and reject every move by Pristina to consolidate its control over the region.

On Friday, hundreds of Kosovo Serbs protested in the flashpoint northern town of Mitrovica against the arrest of an alleged Serb paramilitary leader.

Also on Friday, KFOR issued a statement saying it was "unclear" where exactly the Kosovo police officers were arrested -- whether it was on the territory of Kosovo or Serbia.

It urged both parties "to refrain from provocative rhetoric and to avoid any unilateral act that could escalate the tension in the area".