ZVECAN, Kosovo (AP) - The NATO-led peacekeeping force said on Monday that 25 of its troops were injured in clashes with ethnic Serbs in northern Kosovo after they tried to take over the offices of one of the municipalities where ethnic Albanian mayors took up their posts last week.

The Serbs clashed with NATO troops and Kosovo police in the municipality of Zvecan, 45 kilometers (28 miles) north of the capital. The soldiers fired tear gas and stun grenades to protect the Kosovar officers and disperse protesters, according to witnesses. The assembled Serbs responded by throwing rocks and other hard objects at them.

“Several soldiers of the Italian and Hungarian KFOR contingent were the subject of unprovoked attacks and sustained trauma wounds with fractures and burns due to the explosion of incendiary devices,” said the NATO peacekeepers in a statement.

Some Kosovo police vehicles and one belonging to journalists were damaged and sprayed with Serb nationalist symbols.

Addressing the nation late Monday, Serbian President Aleksandar Vucic said he would spend the night with his troops on the border with Kosovo who were placed on the highest state of alert on his orders last week. He said 52 Serbs were injured in the clashes, three seriously, and four were detained.

“The consequences (of the clashes) are big and grave and the sole culprit is (Kosovo Prime Minister) Albin Kurti,” said Vucic. He referred to the Albanian forces in the north Kosovo as “occupiers.”

“I repeat for the last time and I beg the international community to make sure Albin Kurti sees reason,” Vucic said. “If they don't, I am afraid it will be too late for all of us.”

The violence was the latest incident as tensions soared over the past weekend, with Serbia putting the country's military on high alert and sending more troops to the border with Kosovo, which declared independence from Belgrade in 2008.

Kosovo and Serbia have been foes for decades, with Belgrade refusing to recognize Kosovo's sovereignty.

The United States and the European Union have stepped up efforts to help solve the Kosovo-Serbia dispute, fearing further instability in Europe as Russia's war rages in Ukraine. The EU has made it clear to both Serbia and Kosovo they must normalize relations if they're to make any progress toward joining the bloc.

Russian Foreign Minister Sergei Lavrov described the situation in Kosovo as “worrisome,” blaming the U.S. and NATO for claiming dominance in that part of the world.

“A big `explosion' is brewing in the center of Europe, in the very place where, in 1999, NATO carried out aggression against Yugoslavia,” he said from Nairobi, Kenya, referring to the NATO-led intervention in 1999 that stopped a bloody Serb crackdown against ethnic Albanian separatists.

On Monday, Kosovar police and the NATO-led Kosovo Force, or KFOR, were seen protecting the municipal buildings in Zvecan, Leposavic, Zubin Potok and Mitrovica, four municipalities in the north that held early elections last month.

The votes were largely boycotted by ethnic Serbs, who form the majority in those areas. Only ethnic Albanian or other smaller minority representatives were elected to the mayoral posts and assemblies.

Serbia's prime minister, Ana Brnabic, criticized the international handling of events in Kosovo, saying that KFOR was “not protecting the people … they are protecting the usurpers,” apparently referring to the new mayors.

“But we must protect the peace. Peace is all we have,” she said.

KFOR has increased its presence in the four northern municipalities. It called on all sides to refrain from actions that could cause escalation and urged both “Belgrade and Pristina to engage in the EU-led dialogue.”

U.S. Ambassador Jeff Hovenier met with President Vjosa Osmani and then together with other western powers' ambassadors - the U.S., France, Italy, Germany and the U.K. known as the Quint - with Prime Minister Albin Kurti, urging him to take steps to de-escalate the situation and reduce tensions.

Last Friday, ethnic Serbs in northern Kosovo tried to block recently elected ethnic Albanian officials from entering municipal buildings. Kosovo police fired tear gas to disperse the crowd and let the new officials into the offices.

The U.S. and the EU condemned Kosovo's government for using police to forcibly enter the municipal buildings.

The conflict in Kosovo erupted in 1998 when separatist ethnic Albanians rebelled against Serbia's rule, and Serbia responded with a brutal crackdown. About 13,000 people, mostly ethnic Albanians, died. NATO's military intervention in 1999 eventually forced Serbia to pull out of the territory. Washington and most EU countries have recognized Kosovo as an independent state, but Serbia, Russia and China haven't.

Llazar Semini reported from Tirana, Albania. Jovana Gec contributed to this report from Belgrade, Serbia; Jim Heintz from Tallinn, Estonia.