Violence escalated in northern Kosovo, where local Serb protesters clashed with police and later with NATO-led peacekeepers, leaving dozens of injured.

The force, known as KFOR, said 19 of its Hungarian and 11 Italian soldiers who were “countering the most active fringes of the crowd” on Monday became “the subject of unprovoked attacks and sustained trauma wounds with fractures and burns due to the explosion of incendiary devices.” None of the 30 injured were in life-threatening condition, according to NATO.

More than 50 local Serbs were injured in the flare up, Serb President Aleksandar Vucic told reporters in Belgrade.

It was the worst violence in years in the tense northern area adjacent to Serbia where Vucic reiterated his pledge to protect the Serb minority that accounts for less than 7% of Kosovo’s 1.8 million people.

“We will not tolerate the pogrom of Serbs,” he said, urging the ethnic kin in Kosovo to avoid confrontation with the multinational, NATO-led peacekeeping troops. “I beg Serbs in Kosovo not to engage in confrontation with NATO.”

Serbia’s authorities, including the army, remain in contact with the military alliance to defuse tension and prevent further violence, he said.

Earlier Monday, 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 erupted as Serb protesters tried to block newly elected ethnic-Albanian mayors from reaching their offices.

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

“To avoid the clashes between the parties and to minimize the risk of the escalation, KFOR peace-keepers prevented threats to the lives of Kosovo Serbs and Kosovo Albanians,” NATO said in emailed statement. “Both parties need to take full responsibility for what happened and prevent any further escalation, rather than hide behind false narratives.”

The international force known as KFOR has been deployed in Kosovo since the 1998-99 war between Serbs and ethnic Albanians over Kosovo.

The flareup imperils a European Union-brokered, US-supported plan for the Balkan neighbors to normalize 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.

Vucic said four local prominent Serbs were arrested in Kosovo and that his country was in talks with KFOR negotiating their release.

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

Tensions also closed down schools in the Serb-populated areas of Kosovo, local Serb authorities said in a live broadcast.

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.

(Updates number of NATO troops injured in second paragraph)