Kosovo police clashed with ethnic-Serb protesters in towns in northern Kosovo after activists tried to prevent newly elected ethnic-Albanian mayors from taking office.

Kosovo Police used teargas to disperse crowds blocking mayors from entering local government buildings, authorities said in a Facebook post Friday. The protests followed local elections last month that the ethnic-Serb community boycotted and called invalid.

It was the worst escalation in tensions this year between Kosovo’s ethnic-Albanian majority and the minority Serbs. The flareup imperils an EU-brokered, US-supported plan for the Balkan neighbors to normalize relations, which have threatened to spiral again into violence since Kosovo broke away from Serbia after they fought a 1998-1999 war.

Serbian President Aleksandar Vucic reacted by immediately putting his nation’s army on high alert and ordered movement of an unspecified number of units closer to the border with Kosovo, state broadcaster RTS reported. He took similar steps last year when recurring tensions in Kosovo near the border nearly lapsed into fighting.

Serbian Defense Minister Milos Vucevic said the military is acting on the president’s orders because the situation in Kosovo has become “dramatic.”

He accused Kosovo’s ethnic Albanian leaders of “terrorizing” the Serb minority. Serbs account for less than 7% of Kosovo’s mostly ethnic-Albanian population of 1.8 million.

Blerim Vela, chief of staff for Kosovo President Vjosa Osmani, accused Serbia of exacerbating the incident.

“Serbia’s pyromaniac regime has ordered its military to mass on the border with Kosovo,” Vela said in a tweet.

Following a joint appeal from the European Union, the UK, the US, France, Germany and Italy that the mayors avoid deepening tensions via overt displays of authority, the officials were sworn in during low-key ceremonies over the last week.

But when the officials tried to enter their offices, hundreds of protesters barred their way in the municipalities of Zvecan, Leposaviq and Zubin Potok, police said.

After the crowds were told to disperse, some people hurled objects and gunfire was heard. Five officers were lightly wounded in the clashes, police said.

Ten Serb protesters were injured, the Belgrade-based Tanjug news service reported. Kosovo Premier Albin Kurti’s spokesman denied information that protesters were injured and said the mayors were able to make it to their new offices with police help.

“The central government has a constitutional and legal duty to enable new mayors to enter their offices after taking their oaths,” spokesman Perparim Kryeziu said by phone.

The US condemned the crackdown by Kosovar authorities, with Ambassador to Kosovo Jeff Hovenier saying in a tweet that “today’s violent measures should immediately be halted.”

About 3,800 NATO peacekeepers remain deployed in Kosovo more than two decades after the war, ensuring that neither side reignites armed conflict. KFOR, as the force is known, wasn’t involved in Friday’s events, according to Kryeziu.

The government in Belgrade refuses to recognize Kosovo as a country, and rejects its 2008 split from Serbia — a key sticking point that is blocking both countries’ efforts to join the EU.

Western envoys have sought to defuse the dispute for years, without success. They have intensified those efforts again after Russia attacked Ukraine.

(Updates with Kosovo premier’s comments from 12th paragraph)