At least one police officer was killed and another wounded in an overnight attack in Kosovo's Serb-populated north, with Kosovo Prime Minister Albin Kurti describing the incident as a terrorist act.
Kurti linked the attackers to Serbia, which denies Kosovo's independence and is maintaining links with ethnic Serbs in the area.
"Organized crime, which is politically, financially and logistically supported from Belgrade, is attacking our state," Kurti wrote on his Facebook page.
Kurti said the attack on the police was "still ongoing."
"The attackers are professionals wearing masks and heavily armed."
At a press conference on Sunday, Kurti said that "at least 30" professional gunmen were "surrounded by our police forces" in and around an Orthodox monastery near the village of Banjska. He urged the suspects to surrender.
According to media reports, Kosovo police blocked at least two border crossings into Serbia.
What do we know about the attack?
The overnight attack took place in Banjska, near the Serb town of Leposavic in northern Kosovo. According to police, the officers noticed that a bridge leading up to a local village was blocked by two trucks with no license plates.
After three patrols moved to the trucks to investigate, the attackers opened fire at them from multiple positions and also used grenades.
Eyewitnesses described the events as a "small war" that started in the early hours of the morning, with gunfire continuing for several hours.
"A string of gunshots, then silence, then gunshots, blasts," one of the locals told local KoSSev news outlet.
What is happening at the monastery?
Early on Sunday, KoSSev said its sources from inside the monastery said no attackers were, or had been, at the site. The source said a group of 50 pilgrims from Serbia's Novi Sad was staying at the monastery.
Later on Sunday, the monastery issued a statement saying that a group of armed, masked individuals used an armored car to crash through the monastery gate. The intruders moved across the grounds, and gunshots were occasionally fired, the statement said. The monastery did not say if the cars belonged to the police or the gunmen. They also said monastery residents and pilgrims locked themselves inside the monastery for safety.
In the statement, the monastery officials also condemned the attack on the Kosovo police.
Separately, Kosovo police official Veton Elshan told the AFP news agency by phone that police officers could see "armed people in uniforms ... they are firing on us and we are firing back."
The incident is likely to inflame ethnic tensions between predominantly Orthodox Serbs and predominantly Muslim Albanians in Kosovo.
US envoy condemns 'orchestrated' attacks
Kosovo President Vjosa Osmani blamed "Serbian criminal groups" for the attack and said Kosovo was united "against Serbia's destabilizing attempts."
Most of Kosovo's territory is populated by ethnic Albanians. The Balkan nation was once a part of Serbia, but the Serb forces were driven out in 1999 following an uprising by Albanian guerrilla fighters and the subsequent NATO bombing. The remaining Serb population is now concentrated in northern Kosovo, near the Serbian border.
There was no immediate official response from Belgrade, although Serbian Parliamentary Speaker Vladimir Orlic commented to the Serbian national broadcaster RTS that Kurti had "rushed" into blaming Serbs rather than waiting for more information. Serbian President Aleksandar Vucic was expected to address the media later Sunday.
Meanwhile, US Ambassador in Pristina Jeff Hovenier decried the "orchestrated, violent attacks" on the police and called on perpetrators to "immediately cease" their actions.
The EU's top diplomat Josep Borrell also called for perpetrators to face justice.
"I condemn in the strongest possible terms the hideous attack by an armed gang against Kosovo police officers in Banjska/Banjske in the north of Kosovo," he said.
Ethnic tensions have flared up multiple times in recent years, including protests sparked by a disputed election in May which left dozens of international peacekeepers injured. In June, Serbia arrested three Kosovo police officers, claiming they were deep inside Serbia's territory. Kosovo officials said the men were kidnapped from within Kosovo. The policemen were eventually released, reports UNB.