The White House has said there is an “unprecedented” buildup of Serbian troops and armour along the Kosovo border and called on Belgrade to withdraw them immediately.
The Nato peacekeeping force in Kosovo, Kfor, has been reinforced with British troops and the Biden administration said it was consulting with allies to ensure Kfor’s posture “matches the threat”.
“We are monitoring a large Serbian military deployment along the border with Kosovo that includes an unprecedented staging of advanced Serbian artillery, tanks and mechanized infantry units,” the spokesperson for the US national security council, John Kirby, said on Friday.
“This is a very destabilising development, which has occurred over the past week, and we are calling on Serbia to withdraw forces from the border and lower tensions.”
The national security adviser, Jake Sullivan, called the Kosovan prime minister, Albin Kurti, to discuss the escalation. The secretary of state, Antony Blinken, spoke to the Serbian president, Aleksandar Vučić, and called for “immediate de-escalation” and a return to his previous agreement to normalise relations with Kosovo.
Vučić described his conversation with Blinken as long and difficult. The Serbian president told the Tanjug news agency that he had denied “falsehoods” about the high level of Serbian combat readiness, claiming: “We do not have half as many forces as we had two or three months ago.”
The US warnings come at the end of a week of high tension, beginning with an ambush by well-armed Serb paramilitaries on a Kosovan police patrol, in which a policeman was killed. Three Serb gunmen were killed in the ensuing battle, near the village of Banjska.
The armed group was led by Milan Radoičić, the deputy leader of Serb List, a Belgrade-backed party representing the Serb minority in northern Kosovo. Through a lawyer, Radoičić said he was responsible for the shootout with Kosovan police, but did not explain the source of the modern weapons Serb paramilitaries had been carrying.
The Kosovan government produced a document purporting to show that a grenade launcher the group had been carrying had been given to them by the Serbian army, and officials in Pristina expressed concern Sunday’s gunfight was intended to provide a pretext for a Serbian military intervention in northern Kosovo.
Related: The flare-up of violence in Kosovo shows the folly of the west’s appeasement of Serbia
Serbia declared a day of mourning for the three dead Kosovo Serbs, and Vučić falsely claimed Kosovo forces were conducting a campaign of “brutal ethnic cleansing” against ethnic Serbs.
“This reaction by the White House seems similar to the warnings we saw before Russian troops entered Ukraine,” Donika Emini, the executive director of the Kosovan NGO alliance CiviKos Platform, said, adding that it indicated “conflict is inevitable”.
Emini said one possible Serbian objective was to force a withdrawal of Kosovan police from northern Kosovo, and oblige Kfor to take back full security control in the flashpoint region, further eroding the independence and sovereignty of the former Serbian province.
The UK Ministry of Defence announced on Friday it was transferring to Kfor command of a battalion of the Royal Princess of Wales regiment, which is in the region for a training exercise, to provide support if required.