The US says it is monitoring a large Serbian force along the border with neighbouring Kosovo as tension rises between the two Balkan nations.
What is happening?
Belgrade has deployed advanced “artillery, tanks, mechanised infantry units”, according to the White House.
The purpose of the Serbian build-up is not yet clear but it was a matter of concern, said National Security Council spokesman John Kirby.
What has sparked the tension?
It comes after clashes on Sunday when heavily armed Serb gunmen ambushed a patrol a few kilometres from the Serbian border, killing a Kosovan police officer.
Several dozen assailants later barricaded themselves in a monastery, sparking an hour-long firefight in which three gunmen were killed and three were arrested.
Mr Kirby said the attack had a “very high level of sophistication”, involving about 20 vehicles, “military-grade” weapons, equipment and training.
Kosovo's government has accused Belgrade of backing the operation.
Serbia and the main Serb political group in Kosovo have proclaimed public mourning for the Serbs killed in the battle.
What are the roots of the conflict?
Kosovo, a Muslim majority region, broke away from Serbia in a bloody war in 1998-1999 and declared independence in 2008, a status Belgrade and Russia have refused to recognise.
Russia has traditionally supported Serbia, with which it has close religious and cultural ties.
But about 50,000 Orthodox Serbs in the north have never accepted Kosovo 's independence and still see Belgrade as their capital.
Serbia, which has not recognised its former province's independence, blames Kosovo for precipitating violence by mistreating ethnic Serb residents.
What is Nato's role?
There are about 4,500 peacekeepers stationed in Kosovo, a country of 1.7 million people with little industry and where crime and corruption are rampant.
Nato said it would be “increasing its presence” of its peacekeeping force known as KFOR following the recent violence.
The organisation said in a statement that it had “authorised additional forces to address the current situation” but did not immediately specify how many or from which countries.
On Friday, Britain's Ministry of Defence said it had transferred command of the 1st Battalion of the Princess of Wales's Royal Regiment.
It said the battalion had recently arrived in the region for a long-planned training exercise.
What is the potential political fallout?
Officials in Washington have called for “immediate de-escalation and a return to dialogue”.
US Secretary of State Antony Blinken called Serbian President Aleksandar Vucic to discuss the issue.
Meanwhile, US National Security Adviser Jake Sullivan spoke to Kosovo's Prime Minister in a bid to reduce tension.
The Kremlin said on Monday it was closely monitoring what it called a “potentially dangerous” situation in Kosovo.
