Peacekeepers had to use stun grenades to repel Serb protesters after they threw rocks and stun grenades at the soldiers in the northern town of Zvecan, according to Koha Ditore, a local newspaper in Kosovo.

About 25 soldiers were injured, Nato said in a statement, rebuking “unprovoked attacks” against its soldiers. Among them were 11 Italians, Foreign Minister Antonio Tajani said in a tweet. Most of the remaining injured soldiers were Hungarians.

More than 50 locals were injured in the flare up, regional broadcaster N1 reported, citing chief of hospital in the norther town of Mitrovica, Zlatan Elek.

Earlier in the day, Kosovo’s mostly ethnic-Albanian police forces used pepper spray in response to tear gas hurled by hundreds of ethnic-Serb demonstrators who tried to block access of officials to municipal buildings in Serb-dominated towns.

The escalation of violence — the worst this year — erupted as Serb protesters tried to block newly elected ethnic-Albanian mayors from reaching their offices. The clashes have raised fears that a conflict could reignite between the Balkan neighbors.

“Such attacks are totally unacceptable,” Nato said in the statement. “KFOR will take all necessary actions to fulfill its UN mandate.”

The international force known as KFOR, which has been deployed in Kosovo after the 1998-99 war between Serbs and ethnic Albanians over Kosovo, said in a statement.

The flare-up imperils a European Union-brokered, US-supported plan for the Balkan neighbours to normalise relations. KFOR urged the governments of Serbia and Kosovo to engage in the dialog to reduce tensions.

Following Friday’s clashes, the US and top European allies condemned Kosovo’s government for what they said were actions that provoked the unrest.

The clashes erupted when the mayors tried to access their offices against the recommendation of US and EU mediators. The officials had been elected in a local ballot in April that Serbs boycotted and called invalid.

Serbia’s army remained on high alert, a status ordered by President Aleksandar Vucic on Friday. He took similar steps last year when recurring tensions in Kosovo near the border nearly lapsed into fighting.

Schools in the Serb-populated areas of Kosovo were also closed on Monday, Zvecan’s former Serb mayor, Dragisa Milovic, said in a live broadcast from the town.

The government in Belgrade refuses to recognize Kosovo as a country and rejects its 2008 split from Serbia — a 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 intensified those efforts again after Russia attacked Ukraine.

(With assistance from Jan Bratanic.)