The NATO-led KFOR peacekeeping force on Monday said that 25 of its troops were injured in the clashes with ethnic Serbs in northern Kosovo who were trying to take over the offices of one of the municipalities where ethnic Albanian mayors took up their posts last week.

The Serbs started clashing with the police in the morning in the municipality of Zvecan, 45 kilometers (28 miles) north of the capital, Pristina. In the afternoon, KFOR soldiers called on Serbs to clear the way for two vehicles from the Kosovar special police forces.

"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 a KFOR statement.

Some Kosovo police vehicles and one belonging to journalists were damaged. Pictures showed graffiti with Serb nationalist symbols sprayed over them.

The violence was the latest incident as tensions soared over the past week, 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.

"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.

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.

Police said that Serbs gathered early Monday morning at three of the municipal buildings — in Zvecan, Leposavic and Zubin Potok but not in northern Mitrovica. In Zvecan, they tried to enter using violence and tear gas. The situation remains tense in Zvecan, a police statement said.

Serbia's prime minister, Ana Brnabic, criticized the international handling of the 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.

Defense Minister Milos Vucevic said the Serbian army was wrapping up its deployment following the decision to raise the alert level. Vucevic, who said he was hoping for a political solution, also criticized KFOR, saying that their position "looks like they are protecting the police from unarmed people."

Serbs say they want both the new mayors, whom they called "illegal and illegitimate sheriffs," to resign and leave the offices, and special police to leave northern Kosovo, according to Goran Rakic, a local Serb politician.

KFOR increased its presence in the four municipalities, including Mitrovica. 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.

They suggested the newly elected mayors work in other buildings, not from the municipality buildings.

"Peaceful protest needs to remain peaceful," said Hovenier, denouncing the Serbs' violence.

EU Ambassador Tomas Szunyog denounced the attack on journalists' cars saying that "journalists must be allowed to conduct their work without fear for their safety."

Last Friday, ethnic Serbs in northern Kosovo, who are a majority in that part of the country, 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.

More than a dozen Serbs and five Kosovar police officers were injured. Serbian troops on the border with Kosovo were put on high alert the same day.

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

At a rally Friday evening in Belgrade with his supporters, Serbian President Aleksandar Vucic said, "Serbia won't sit idle the moment Serbs in northern Kosovo are attacked."

However, any attempt by Serbia to send its troops over the border would mean a clash with NATO-led troops stationed there.

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.