WASHINGTON, Sep 29 (AFP) - The United States urged Belgrade to pull its forces back from the border with Kosovo on Friday after detecting what it called an "unprecedented" Serbian military build-up.

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

The violence -- in which a Kosovo police officer 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 spokesman 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 added: "We are calling on Serbia to withdraw those forces from the border."

The build-up took place within the last week but its purpose was not yet clear, Kirby said.

US Secretary of State Antony Blinken had telephoned Serbian President Aleksandar Vucic to urge "immediate deescalation and a return to dialogue," he added.

National Security Advisor Jake Sullivan also spoke with Kosovo Prime Minister Albin Kurti and "expressed concern about Serbian military mobilizations," according to a readout of the call.

The pair also "discussed the EU-facilitated Dialogue between Kosovo and Serbia, which Mr. Sullivan underscored was the only long-term solution to ensuring stability throughout Kosovo," the readout said.

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

Photo from AFP