washington - The United States called on Belgrade to pull its forces back from the border with Kosovo on Friday after detecting what it called an unprecedented Serbian military buildup.

Serbia deployed sophisticated tanks and artillery on the frontier after deadly clashes erupted at a monastery in northern Kosovo last week, the White House said.

The violence in which a Kosovo policeman and three Serb gunmen were killed marked one of the gravest escalations for years in Kosovo, a former Serbian breakaway province.

'We are monitoring a large Serbian military deployment along the border with Kosovo,' White House National Security Council spokesperson John Kirby told reporters. 'That includes an unprecedented staging of advanced Serbian artillery, tanks, mechanized infantry units.'

'We believe that this is a very destabilizing development,' he said. 'We are calling on Serbia to withdraw those forces from the border.'

The buildup happened within the past week, but its purpose was not yet clear, Kirby said.

U.S. Secretary of State Antony Blinken had telephoned Serbian President Aleksandar Vucic to urge an 'immediate de-escalation and a return to dialogue.'

And White House national security adviser Jake Sullivan spoke to Kosovo's prime minister.

Serbia's Vucic did not directly deny there had been a recent buildup but rejected claims that his country's forces were on alert.

'I have denied untruths where they talk about the highest level of combat readiness of our forces, because I simply did not sign that and it is not accurate,' Vucic told reporters. 'We don't even have half the troops we had two or three months ago.'

'Worrisome'

Serbia said on Wednesday that the defense minister and head of the armed forces had gone to visit a 'deployment zone' but gave no further details.

The clashes on Sunday began when heavily armed Serb gunmen ambushed a patrol a few kilometers from the Serbian border, killing a Kosovo police officer.

Several dozen assailants then barricaded themselves at an Orthodox monastery, sparking an hourlong firefight in which three gunmen were killed and three were arrested.

Kosovo's government has accused Belgrade of backing the entire operation. A member of a major Kosovo Serb political party admitted to leading the gunmen, his lawyer said Friday.

Kirby said the attack had a 'very high level of sophistication,' involving around 20 vehicles, 'military-grade' weapons, equipment and training.

'It's worrisome,' he said. 'It doesn't look like just a bunch of guys who got together to do this.'

Peacekeeping force expected to grow

NATO would be 'increasing its presence' of its peacekeeping force known as KFOR following the attack, Kirby added.

In Brussels, NATO chief Jens Stoltenberg confirmed that the U.S.-led alliance was ready to boost the force to deal with the situation.

Kosovo broke away from Serbia in a bloody war in 1998-99 and declared independence in 2008 - a status Belgrade and Moscow have refused to recognize.

It has long seen strained relations between its ethnic Albanian majority and Serb minority, which have escalated in recent months in northern Kosovo.