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, according to Reuters.
"We urge the institutions in Kosovo to de-escalate immediately & 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.
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.
RHM/PR