The White House urges Serbia to withdraw its forces from the border with Kosovo.

Belgradeto pull back their forces from the border with Kosovo on Friday after discovering what they called an “unprecedented” military buildup of forcesSerbia.

Whitehousesaid the reinforcement occurred last week, but its purpose is not yet clear. Serbia's leader dismissed accusations that his country's forces were on alert. NATO has confirmed that the size of its peacekeeping force in Kosovo will increase.

Serbiadeployed advanced tanks and artillery to the border after deadly clashes erupted last week in a monastery in northern Kosovo, the White House warned. The violence, which killed a Kosovo policeman and three Serb gunmen, was one of the worst escalations in recent years in Kosovo, a former Serbian breakaway province.

“We are monitoring a large military deployment along the border with Kosovo.”- said a spokesman for the White House National Security CouncilhousesJohn Kirby to reporters.“This includes the unprecedented deployment of modern Serbian artillery, tanks, mechanized infantry units. We believe this is a very destabilizing development.”

“We urge.Serbiato withdraw its forces from the border.” The deployment happened last week, but its purpose is not yet clear, Mr. Kirby said. Secretary of StateU.S.Antony Blinken called Serbian President Aleksandar Vucic to call for “an immediate reduction in tensions and a return to dialog,” he added. National security aide Jake Sullivan also spoke with Kosovo Prime Minister Albin Kurti and “expressed concern about Serbia's military mobilization,” a report on the call reads.

Mr.

Vucic, the Serbian leader, does not deny that there has been a recent reinforcement, but dismissed claims that his country's forces are on alert. “I denied the false information about what they say about the highest level of readiness of our forces, because I just didn't sign that, and it's not true,” Mr. Vucic told reporters. “We don't even have half of the troops we had two or three months ago.”

Last week's clashes began when Serbian gunmen killed a Kosovo policeman. Serbia said Wednesday that the defense minister and the armed forces chief had gone to visit the “deployment zone” but did not disclose additional details.

NATO's peacekeeping force in Kosovo, known as KFOR, will “increase its presence” after the attack, Mr. Kirby added. In Brussels, NATO Secretary General Jens Stoltenberg reiterated that the U.S.-led alliance was ready to increase force to deal with the situation.

Kosovo seceded from Serbia in a bloody war in 1998-1999 and declared independence in 2008, a status that Belgrade and Moscow refuse to recognize. Kosovo has long had tensions between its Albanian majority and Serb minority, which have escalated in recent months in northern Kosovo.