LEPOSAVIC, Kosovo NATO peacekeeping soldiers formed security cordons around three town halls in Kosovo on Monday to keep back Serbs protesting at ethnic Albanian mayors taking office in a Serb majority area after elections they boycotted.

In Zvecan, one of the towns, Kosovo state police - staffed entirely by ethnic Albanians after all Serbs quit the force last year - sprayed pepper gas to repel a crowd of Serbs who broke through a security barricade and tried to force their way into the municipality building, witnesses said.

In Leposavic, close to the border with Serbia, U.S. peacekeeping troops in anti-riot gear placed barbed wire around the municipality building to protect it from hundreds of angry Serbs gathering nearby.

NATO peacekeepers also blocked off the town hall in Zubin Potok to protect it from angry local Serbs, witnesses said.

Serbs, who form a majority in Kosovo's north, have never accepted its 2008 declaration of independence from Serbia and still see Belgrade as their capital more than two decades after the Kosovo Albanian uprising against repressive Serbian rule.

Ethnic Albanians make up more than 90% of the population in Kosovo as a whole, but northern Serbs have long demanded the implementation of an EU-brokered 2013 deal for the creation of an association of autonomous municipalities in their area.

Serbs refused to take part in local elections in April and ethnic Albanian candidates won the mayoralties in four Serb-majority municipalities - including North Mitrovica, where no incidents were reported on Monday - with a 3.5% turnout.

Serbs demand that the Kosovo government remove ethnic Albanian mayors from town halls and allow local administrations financed by Belgrade return to their duties.

On Friday, three out of four mayors were escorted into their offices by police, who were pelted with rocks and responded with tear gas and water cannon to disperse the protesters.

On Sunday NATO Secretary General Jens Stoltenberg called on the Kosovo government to tone down tensions with Serbia. "Pristina must de-escalate & not take unilateral, destabilising steps," Stoltenberg said in a tweet.

Following a phone call with EU foreign policy chief Josep Borrell Kosovo Prime Minister Albin Kurti tweeted: "Emphasized that elected mayors will provide services to all citizens."

NATO peacekeepers were deployed in Kosovo after the 1999 NATO bombing campaign that drove the Serbian military and security police out of Kosovo, ending a brutal counter-insurgency campaign.

(This story has been refiled to add 'Kosovo' in the headline)

(Reporting by Fatos Bytici; writing by Ivana Sekularac; editing by Mark Heinrich)