Serbia's army remained on the "highest level of alert" near the border with Kosovo, the Balkan country's presidency said Saturday, as NATO urged Kosovo to dial down tensions with Serbia, a day after its government forcibly accessed municipal buildings to install mayors in ethnic Serb areas in the north of the country and clashes broke out between ethnic Serbs and Kosovo police.

The resulting clashes on Friday between Kosovan police and protesters opposed to the ethnic Albanian mayors prompted Serbia to put its army on full combat alert and to move units closer to the border.

"We urge the institutions in Kosovo to de-escalate immediately and call on all parties to resolve the situation through dialogue," said Oana Lungescu, a spokeswoman for the transatlantic military alliance, in a Twitter post.

She said KFOR, the 3,800-strong NATO-led peacekeeping mission in Kosovo, would remain vigilant.

Things were still tense in the north part of the country where heavily armed police forces in armored vehicles were guarding municipality buildings.

Kosovo's Prime Minister Albin Kurti defended police actions in escorting the new mayors the previous day.

"It is the right of those elected in democratic elections to assume office without threats or intimidation. It is also the right of citizens to be served by those elected officials," Kurti said on Twitter on Saturday.

U.S. Secretary of State Antony Blinken on Friday criticized Kurti's government for its actions in the north, saying they "unnecessarily escalated tensions, (were) undermining our efforts to help normalize relations between Kosovo and Serbia and will have consequences for our bilateral relations with Kosovo."

Almost a decade after the end of a war there, Serbs in Kosovo's northern region do not accept Kosovo's 2008 declaration of independence from Serbia and still see Belgrade as their capital.

Ethnic Albanians form more than 90% of the population in Kosovo, with Serbs only the majority in the northern region.