NATO peacekeeping soldiers formed security cordons around four town halls in Kosovo on Monday to keep back Serbs protesting ethnic Albanian mayors taking office in a Serb majority areas after they had boycotted elections.

In Zvecan, one of the towns, Kosovo 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, 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.

"This morning, the NATO-led Kosovo Force mission (KFOR) has increased its presence in four municipalities of northern Kosovo following the latest developments in the area," a KFOR statement said.

"In line with its mandate, KFOR is ready to take all necessary actions to ensure a safe environment in a neutral and impartial manner," it said, adding that KFOR's commander was in close contact with the security organs of Kosovo and Serbia.

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. Serbia also does not recognize an independent Kosovo.

Ethnic Albanians make up more than 90 percent of the population in Kosovo as a whole, but northern Serbs demand the implementation of a decade-old EU-brokered 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 with a 3.5-percent turnout.

Serbs have called on Pristina to remove ethnic Albanian mayors from town halls and allow local administrations financed by Belgrade to return to their duties.

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

The United States and its allies, who have strongly backed Kosovo's secession from Serbia, rebuked Pristina on Friday for escalating tensions with Serbia, saying the use of force to install mayors in majority Serb areas undercut efforts to normalize relations.

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

(With input from Reuters)