Gunmen holed up in a northern Kosovo monastery have clashed with authorities, according to the country’s prime minister, hours after a police officer was killed when a patrol was hit by an ambush involving firearms and explosives. Albin Kurti said the group of at least 30 heavily armed gunmen were surrounded by authorities near the village of Banjska, where the patrol had been ambushed earlier on Sunday, and called for their surrender. The attack and subsequent fighting mark one of the gravest escalations in Kosovo in years, after months of mounting tensions and stalled talks between the country’s government and Serbia. Kurti said at a press conference: “There are at least 30 professional, military or police armed people who are surrounded by our police forces and whom I invite to surrender to our security agencies.” The Serbian Orthodox church confirmed that gunmen had stormed a monastery in Banjska, where pilgrims from the northern Serbian city of Novi Sad were staying. Pictures released by Kosovan authorities showed several heavily armed gunmen in uniform barricading themselves at the monastery. The Kosovo police official Veton Elshani told Agence France-Presse by phone from Banjska: “We can see armed people in uniforms … They are firing on us and we are firing back.” Police later said in a statement that at least three attackers had been killed and one arrested. Another four civilian suspects carrying radio equipment and weapons were also arrested. Kosovo police also confirmed that the Jarinje and Brnjak border crossings between Serbia and Kosovo had been closed as a result of the clashes. Kurti’s comments at the press conference came hours after he called the ambush that killed one police officer and wounded another an act of terrorism and blamed the Serbian government. “Organised crime with political, financial and logistical support from officials in Belgrade is attacking our country,” Kurti wrote on social media. The Nato-led KFOR mission said its forces were present in the area and “standing ready to respond if required”. According to Kosovan law, government authorities are not allowed to enter Orthodox properties, including churches and monasteries, without first receiving permission from the Serbian Orthodox church. Serbia’s president, Aleksandar Vučić, was expected to address the media on Sunday evening. According to the Serbian government’s office for Kosovo, the president’s press conference would “debunk all the lies and hoaxes of Albin Kurti, the creator of chaos and hell in Kosovo”. The attack comes more than a week after talks between the leaders of Serbia and Kosovo centred on improving ties failed to make a breakthrough during EU-mediated negotiations in Brussels. The EU has been trying for years to resolve the long-running dispute between the Balkan neighbours that has soured relations since their war more than two decades ago. Brussels believed it had broken the impasse in March by hammering out a plan to normalise ties, but since then there has been minimal progress. The EU’s top diplomat, Josep Borrell, condemned Sunday’s attack, calling the targeting of police “hideous”. “The responsible perpetrators must face justice,” Borrell wrote on social media. His office later called it a “cowardly terrorist attack”. Tensions in northern Kosovo have been smouldering for months, after the Pristina government’s decision to install ethnic Albanian mayors in four Serb-majority municipalities in May. The move triggered one of the worst bouts of unrest in the north in years. Demonstrations followed, as well as the arrest of three Kosovan police officers by Serbia and a violent riot by Serbian protesters during which more than 30 Nato peacekeepers were injured. Kosovo remains overwhelmingly populated by ethnic Albanians, but in the northern stretches of the territory near the border with Serbia, ethnic Serbs are the majority in several municipalities. The tussle in the north is the latest in a long list of incidents to rock the area since Kosovo declared independence from Serbia in 2008. That was nearly a decade after Nato forces helped push Serbian troops from the former province during a war that killed about 13,000 people. Belgrade – along with its key allies China and Russia – has refused to recognise Kosovo’s independence, in effect preventing it from having a seat at the United Nations. During an address to the UN general assembly last week, Vučić accused the west of hypocrisy. He said its recognition of Kosovo’s independence was based on the same justification as Russia’s war on Ukraine.