ZVECAN, Kosovo (AP) — Ethnic Serbs in northern Kosovo clashed with police at a municipal building on Monday as they tried to take over one of the local communes where ethnic Albanian mayors entered last week with the help of authorities.

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 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 to advance in their intentions to join the bloc.

On Monday, Kosovar police and the NATO-led Kosovo Force, or KFOR, were seen protecting the municipality buildings in Zvecan, Leposavic, Zubin Potok and Mitrovica, four communes in the north that held early elections last month. They were largely boycotted by ethnic Serbs, who form the majority in those areas. Only ethnic Albanian or other smaller minority representatives were elected in the mayoral posts and assemblies.

Police said that Serbs gathered early in the morning at three of the commune buildings — in Zvecan, Leposavic and Zubin Potok but not in northern Mitrovica. In Zvecan, they tried to enter violently using tear gas in their efforts to get into the public buildings. Police responded with tear gas spray, a statement said.

Serbia’s prime minister, Ana Brnabic, criticized the international handling of the events in Kosovo, saying that KFOR is “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 is wrapping up deployment following the decision to raise combat readiness and will be ready to “fulfill any task and any order.” Vucevic said that he is hoping for a political solution to the crisis. He 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 offices, and special police to leave northern Kosovo, according to Goran Rakic, a Serb politician from northern Kosovo, adding that the demands were also sent to KFOR and international embassies.

Dragisa Milovic, another Serb politician in northern Kosovo, said “people have gathered to peacefully and democratically convey that we are worried about the situation and our future,” describing the situation as “pure occupation.”

KFOR said it has increased its presence in the four municipalities, including Mitrovica, “to ensure a safe and secure environment and freedom of movement for all communities in Kosovo.” It called on all sides “to refrain from actions that could inflame tensions or cause escalation” and urged both “Belgrade and Pristina to engage in the EU-led dialogue to reduce tensions and as the only way to peace and normalization.”

After meeting with Kosovar President Vjosa Osmani on Monday, U.S. Ambassador Jeff Hovenier repeated a call from Western powers — the U.S., France, Italy, Germany and the U.K. — “to avoid circumstances where public officials are accessing public buildings through the use of force.” Together with EU Ambassador Tomas Szunyog, they met with two mayors of northern communes to discuss “how they can fulfill their duties to serve all their citizens.” Two others didn't attend the informal meeting.

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

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.

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

On Sunday evening, France, Germany, Italy, the U.K., the U.S. and EU again issued a statement saying they strongly caution "all parties against other threats or actions which could impact on a safe and secure environment, including freedom of movement, and that could inflame tensions or promote conflict.”

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 troops stationed there.

A 2013 Pristina-Belgrade agreement on forming the Serb association was later declared unconstitutional by Kosovo’s Constitutional Court, which said the plan wasn’t inclusive of other ethnicities and could entail the use of executive powers to impose laws.

The two sides have tentatively agreed to back a EU plan on how to proceed, but tensions still simmer.

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.

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Llazar Semini reported from Tirana, Albania. Jovana Gec contributed to this report from Belgrade, Serbia.