The White House on Friday said it had detected a large build-up of Serbian forces on the Kosovo border and urged Belgrade to withdraw them to avoid destabilising the situation after deadly clashes.
In Brussels meanwhile, Nato said it had authorised additional forces for Kosovo. Britain's Ministry of Defence said it had transferred command of a battalion to the alliance so it could provide support if required.
The killing of a Kosovo police officer and an ensuing gun battle at a monastery in a village close to the Serbian border on Sunday marked one of the gravest escalations in the former breakaway province in years.
“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, mechanised infantry units. We believe that this is a very destabilising development.”
Mr Kirby added: “We are calling on Serbia to withdraw those forces from the border.”
The purpose of the Serbian build-up was not yet clear but it was a matter of concern, he said.
Kosovo, which declared independence from Serbia in 2008 after a guerrilla uprising and 1999 Nato intervention, accuses Serbia of arming and supporting Serb fighters.
Serbia, which has not recognised its former province's independence, blames Kosovo for precipitating violence by mistreating ethnic Serb residents. Serbia and the main Serb political group in Kosovo have proclaimed public mourning for the Serbs killed in the battle.
US Secretary of State Antony Blinken called Serbian President Aleksandar Vucic to urge “immediate de-escalation and a return to dialogue”, Mr Kirby said.
National Security Adviser Jake Sullivan also spoke to Kosovo's Prime Minister.
He sent condolences for the police officer's death and "expressed concern" about the Serbian military buildup.
The White House said the two discussed the EU-facilitated dialogue, which Mr Sullivan underscored "was the only long-term solution to ensuring stability throughout Kosovo".
Nato will be “increasing its presence” of its peacekeeping force known as KFOR, Mr Kirby said, without giving details.
Britain's Ministry of Defence said it had transferred command of the 1st Battalion of the Princess of Wales's Royal Regiment to Nato.
Three Serb gunmen were killed in the hours-long firefight with Kosovo police, after they ambushed a patrol near the village of Banjska and later barricaded themselves in an Orthodox monastery.
A member of a major Kosovo Serb political party has admitted to leading the group of gunmen, his lawyer said on Friday, according to AFP.
Mr Kirby said the attack had a “very high level of sophistication”, involving a high level of manpower, vehicles, equipment and training.
“It's worrisome. It doesn't look like just a bunch of guys who got together to do this,” he said.