PRISTINA, Kosovo — Kosovo on Monday observed a day of mourning for the Kosovar Albanian police officer killed by Serb gunmen who then barricaded themselves in an Orthodox monastery in a siege that further raised tensions as the two wartime foes seek to normalize ties.

Flags were at half-staff on all public buildings in the capital Pristina to mourn Afrim Bunjaku. In the north, where most of Kosovo’s ethnic Serb minority lives in four municipalities around Mitrovica, police were patrolling in search of the armed assailants after they left the monastery.

About 30 gunmen dressed in combat uniforms were involved in the attack, but it is not clear who they are or who is supporting them. Pristina accuses Belgrade of backing the “terrorists,” an accusation Serbia denies, saying they are Serbs from Kosovo protesting the government there.

On Sunday the masked gunmen opened fire on a police patrol at about 3 a.m. (01:00 GMT) in Banjska, a village located 55 kilometers (35 miles) north of Pristina, killing Bunjaku and injuring another officer.

They then used an armored vehicle to break down the gates to the monastery in the village, where they remained in a stand-off with Kosovo police until evening.

The two sides exchanged gunfire sporadically until darkness fell, when the assailants escaped from the monastery on foot.

Three of the attackers were killed and two injured. Another Kosovar police officer was injured in the confrontation near the monastery.

Two of the gunmen and four Serbs discovered nearby with communication equipment were arrested and are being investigated for terrorist acts.

Police seized vehicles used by the gunmen which contained an arsenal of firearms of different calibers, explosives, ammunition and logistics capable of equipping hundreds of persons, according to Kosovo Interior Minister Xhelal Svecla.

“It’s a terrorist, criminal, professional unit that had planned and prepared what they did and who are not a smuggling band but a mercenary structure which is politically, financially and logistically supported by official Belgrade,” sad Prime Minister Albin Kurti.

Serbian President Aleksandar Vucic said the gunmen were local Kosovo Serbs “who no longer want to stand Kurti’s terror.”

Vucic condemned the killing of the Kosovo policeman, but said the clash was the result of “brutal” pressure on Kosovo Serbs by the government there. He denied any involvement by Belgrade.

Vucic also blasted the West and its “hypocrisy” over Kosovo.

“You can kill us all. Serbia will never recognize the independence of Kosovo, that monster creation that you made by bombing Serbia,” Vucic said, referring to the 1999 NATO intervention which led to Kosovo separating from Serbia.

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 international community condemned the “hideous attack.” The European Union and the NATO-led international peacekeeping force in Kosovo are in close contact with Kosovar authorities.

Earlier this month, an EU-facilitated meeting between Kurti and Vucic to normalize ties ended in acrimony. The United States has supported the negotiations and the EU’s position in trying to resolve the ongoing source of tension in the Balkans.

In February, the EU put forward a 10-point plan to end the latest escalation of tensions. Kurti and Vucic gave their approval at the time, but with some reservations that have still not been resolved.

The EU warned both countries that their commitments in February “are binding on them and play a role in the European path of the parties” — in other words, Serbia and Kosovo’s chances of joining the 27-nation bloc.