The United States on Friday called on Serbia to pull its forces back from the border with Kosovo.

The White House warned that Belgrade had deployed tanks and artillery near the border since deadly clashes erupted at a monastery in northern Kosovo last week.

Who was involved in the clashes in northern Kosovo?

On Sunday, three gunmen were killed in a shootout with Kosovar police after they ambushed a patrol near the northern Kosovar village of Banjska. One policeman was killed in the clash, which involved some 30 gunmen.

Ethnic Albanians form over 90% of Kosovo's population, but the northern part of Kosovo is predominantly inhabited by ethnic Serbs.

Also on Friday, Milan Radoicic, a politician belonging to Kosovo's Serb List political party, admitted to organizing the attack, his lawyer said.

Kosovo Prime Minister Albin Kurti said in comments to the Al Jazeera broadcaster that the Kosovo Serb politician was only the "executor" of the attack and accused Belgrade of having planned and ordered it.

What did the US say about Serbia's deployment near Kosovo?

"We are monitoring a large Serbian military deployment along the border with Kosovo," White House National Security Council spokesman John Kirby told reporters.

"That includes an unprecedented staging of advanced Serbian artillery, tanks, mechanized infantry units," he said. "We believe that this is a very destabilizing development."

Kirby said the reason for the military build-up was unclear but that it was a matter of concern.

"We are calling on Serbia to withdraw those forces from the border," he said.

Kirby said NATO would be "increasing its presence" in Kosovo without providing further details.

Referring to the attack on Sunday, Kirby said that it had a "very high level of sophistication" and a high level of equipment and training. "It doesn't look like just a bunch of guys who got together to do this."

NATO says it has authorized additional forces for deployment as part of the KFOR peacekeeping force Deutsche Welle

NATO authorizes additional KFOR forces

Also on Friday, the NATO military alliance said it had authorized additional forces for Kosovo. NATO leads the Kosovo Force (KFOR) peacekeeping mission.

The alliance said it had "authorized additional forces to address the current situation" but did not provide further details.

Later, the British Ministry of Defense said it had transferred one of its battalions to NATO to provide support if required. The battalion is a reserve force for KFOR.

Blinken, Vucic discuss de-escalation

US Secretary of State Antony Blinken spoke with Serbian President Aleksandar in a phone call on Friday.

Blinken and Vucic discussed the importance of taking immediate measures for de-escalation, according to Blinken's spokesman Matthew Miller.

Miller said Blinken "underscored that those responsible for the attacks who are now in Serbia must be held accountable."

Blinken also voiced support for the actions of the NATO-led KFOR peacekeeping mission and the EU's EULEUX mission in responding to the violence.