PRISTINA, Kosovo (AP) — Kosovo police on Friday raided several locations in a tense Serb-dominated area in the north of the country where weekend clashes left four people dead and further strained relations with Serbia.

Police said in a statement that they were conducting searches on five locations in three municipalities in northern Kosovo. A statement said the operation was in connection with Sunday’s shootout between Serb insurgents and Kosovo police in the village of Banjska.

The confrontation was one of the worst since Kosovo declared independence from Serbia in 2008 and Belgrade refused to recognize the split.

About 30 masked men opened fire on a police patrol near Banjska before breaking down the gates of a Serbian Orthodox monastery and barricading themselves inside with the priests and visiting pilgrims. The 12-hour shootout that followed left one police officer and three gunmen dead.

The violence further raised tensions in the Balkan region at a time when European Union and U.S. officials have been pushing for a deal that would normalize ties between Serbia and Kosovo. A NATO bombing campaign on Serb positions in Kosovo and Serbia led to the end of their 1998-99 war. The was left some 10,000 people dead, mostly Kosovo Albanians.

Serbian media said that police on Friday raided a hospital and a restaurant in the Serb-dominated part of the town of Mitrovica, as well as locations in other towns in the area. The local Kossev news agency reported police also confiscated several vehicles.

Kosovo has accused Serbia of direct involvement in the clashes in Banjska, which Belgrade has denied. Kosovo police said they had found huge quantities of weapons and equipment that suggested the insurgents were planning a wider operation.

On Thursday, Kosovo's interior minister, Xhelal Sveçla, told The Associated Press in an interview that Serbia operates training camps for the insurgents and that Kosovo authorities were also investigating Russia's involvement in the violence.

There are fears in the West that Russia, acting through Serbia, may want to destabilize the Balkans and shift at least some of the attention from Moscow’s full-scale invasion of Ukraine. Russia has voiced support for Serbia over the clashes, blaming the West for allegedly failing to protect Kosovo Serbs.

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