PRISTINA, Kosovo (AP) — Kosovo’s interior minister on Thursday accused Serbia of direct involvement in weekend clashes and was investigating the possibility of Russian involvement in the violence that left four people dead and further strained relations between the former wartime foes.

One Kosovo police officer and three gunmen were killed in Sunday’s shootout between Serb insurgents and Kosovo police. Eight people were initially arrested, but four of them have been released from custody because of a lack of evidence.

Kosovo Interior Minister Xhelal Sveçla told The Associated Press in an interview that investigators were looking at evidence linking Russia, an ally of Serbia, to the armed assault. Russian weapons, other equipment and documents suggesting Russian involvement were discovered after the daylong gunbattle, he said.

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. They then broke down the gates of a Serbian Orthodox monastery and barricaded themselves inside with the priests and visiting pilgrims.

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.

“What we know for sure is that they (the insurgents) came from Serbia,” Sveçla said. “Some of them are Kosovo Serbs, who have dual citizenship, Kosovo and Serbian citizenship, that, by our intelligence information, were trained in camps in Serbia.”

“We found some documents which lead us to a suspicion that there were individuals coming from Russia too,” he said. “For the equipment, we have evidence, but for the people we still have only suspicion.”

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.

Serbia’s president, Aleksandar Vučić has denied claims that Serbia was involved in the clash, saying the gunmen were local Kosovo Serbs “who no longer want to withstand the terror” of Kosovo’s ethnic Albanian authorities. Vučić's statements have portrayed the gunmen as heroes, and a national day of mourning was held for the three killed Serbs in both Serbia and northern Kosovo where they represent a majority.

After a meeting on Thursday with Danish Foreign Minister Lars Løkke Rasmussen, Vučić described the situation as “the most complicated and dangerous so far,” blaming the tensions on the policies of Kosovo’s government and urging international help to protect ethnic Serbs.

Serbia’s chief negotiator with Kosovo, Petar Petković, demanded an independent investigation into the killing of the three Serb gunmen, saying they were “brutally” executed after surrendering to Kosovo police. He said that Belgrade has witnesses and evidence, including photographs, but didn’t make them public.

Sveçla said that based on the gathered evidence and drone footage, some of the assailants, who he referred to as “terrorists,” wore face masks throughout the operation.

“Even the members of the group who were arrested didn’t know who they were,” he said. “So they are either high officials from the security sector of Serbia or they might be directly coming from Russia itself.”

The interior minister said that the aim of the operation appeared to be the seizure of northern Kosovo with the ultimate goal of seceding from the rest of the country.

“They had a 100 more uniforms,” Sveçla said. “They were planning to recruit more people within (the communities) where Serbs live ... Kosovo institutions would have to deal with the few hundred terrorists in a scenario which they planned.”

He said that judging from a large cache of weapons found at the scene of the clash, the attackers planned to heavily arm at least a couple of hundred people.

“Uniforms, personal armor, Kalashnikovs, sniper rifles, pistols. All this tells us there was equipment for a couple of hundred people together with logistics. It was equipment and weapons for a whole battalion,” he said.

The danger of a new insurgency isn’t over, Sveçla said.

“We don’t have clashes, but we have information that training of these terrorists continues in Serbia,” he said. “There are still people in Kosovo who are part of this group.”

Serbia and Kosovo, a former province belonging to Belgrade, 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 Serbia 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, Kosovo Prime Minister Albin Kurti and Vučić gave their approval to a 10-point EU plan for normalizing relations, but the two leaders have since distanced themselves from the agreement.

___

Dusan Stojanovic and Jovana Gec contributed to this report from Belgrade, Serbia.