BELGRADE, Serbia (AP) — Serbia’s president demanded Tuesday to have a NATO-led peacekeeping force take over for the national law enforcement agency in northern Kosovo after a daylong shootout between armed Serbs and Kosovar police left one officer and three gunmen dead.

In one of the worst confrontations since Kosovo declared independence from Serbia in 2008, about 30 masked men opened fire on a police patrol near the village of Banjska early Sunday then broke down the gates of a Serbian Orthodox monastery and barricaded themselves with the priests and visiting pilgrims.

The violence further raised tensions in the Balkan region at a time when European Union and U.S. mediators have been pushing for a deal that would normalize ties between former wartime foes Serbia and Kosovo. A NATO bombing campaign on Serb positions in Kosovo led to the end of the 1998-99 war.

Serbian President Aleksandar Vucic met with ambassadors from five Western countries and the EU in Belgrade on Tuesday. He said he wanted the NATO-led Kosovo Force, or KFOR, to take over “all the security matters in the north of Kosovo instead of (Kosovo Prime Minister Albin) Kurti’s police.”

Kurti accused the Serbian government on Sunday of logistically supporting “the terrorist, criminal, professional unit” that fired on Kosovo Police officers. Vucic denied the allegations, saying the gunmen were local Kosovo Serbs “who no longer want to withstand Kurti’s terror.”

KFOR has around 4,500 troops stationed in Kosovo, where ethnic Serbs are concentrated in four northern municipalities. There was no immediate reaction from NATO to Vucic’s request, but it is highly unlikely to be granted because the primary role of the troops is peacekeeping, not policing.

The Western military alliance bolstered the number of troops in Kosovo after clashes with ethnic Serbs there in May left 30 international soldiers wounded.

Vucic, a pro-Russian politician, has often spoken against NATO and its peacekeeping mission in Kosovo, blaming them for allegedly failing to protect minority Serbs from Kosovo Albanian harassment.

The weekend standoff ended when most of the assailants escaped on foot under cover of darkness on Sunday evening. Three of the gunmen were shot and killed by police. U.S. Ambassador to Serbia Christopher Hill said after the meeting with Vucic that the episode was a very serious event.

“And the concern is, of course, that it could get even worse,” he told reporters.

“We really deeply regret the loss of life and frankly, we condemn the killing of the Kosovo police officer,” Hill said. “Beyond that, I think it is very important to know what happened but equally important to move the political and diplomatic process, which I think really needs to be strengthened in the days and months ahead.”

Vucic said in an Instagram post that these are “one of the hardest moments for Serbia.” Earlier, the government proclaimed Wednesday as a day of mourning because of “the tragic events,” referring to the shootout.

A Kosovo Serb party allied with Vučić proclaimed three days of mourning starting Tuesday in Serb-dominated northern Kosovo for the three killed Serb assailants. In a sign of defiance, hundreds of Serbs in the northern Kosovo town of Mitrovica lit candles and laid flowers for the killed Serbs.

On Tuesday in Kosovo’s capital, a court decided to keep three of the six gunmen who were arrested after Sunday’s gunfight in pretrial detention for a month. They are accused of violating the country’s constitution and of terror acts.

Kosovo Interior Minister Xhelal Svecla claimed Tuesday that the leader of the Serb gunmen who killed the Kosovo police officer is Milan Radoicic, a close ally of Vucic’s who was sanctioned by the U.S. and Britain for alleged criminal activities.

Radoicic, a businessman, is deputy leader of a pro-Vucic Kosovo Serb party that has been calling the shots in northern Kosovo.

Svecla posted drone footage on Facebook that allegedly shows Radoicic with a group of uniformed men leaning against a wall of the Orthodox monastery where the gunmen barricaded themselves. The authenticity of the video could not be confirmed.

“Chief criminal Radoicic has been the leader of the terrorist group and of the attack where police officer Afrim Bunjaku was killed,” Svecla said.

The minister also claimed that Radoicic was wounded in the shootout and undergoing treatment at a Belgrade hospital.

Serbia and Kosovo, its former province, have been at odds for decades. Their 1998-99 war left more than 10,000 people dead, mostly Kosovo Albanians. Kosovo unilaterally declared independence in 2008, but Belgrade has refused to recognize the move.

The EU, with the backing of the U.S., has been brokering negotiations between the two sides. In February, Kurti and Vucic gave their approval to a 10-point EU plan for normalizing relations, but the two leaders have since distanced themselves from the agreement.

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Llazar Semini contributed to this report from Tirana, Albania.