One US soldier has been killed and another injured in an apparent insider attack in Afghanistan, the Nato-led mission in the country says.
Initial reports suggest the assailant in Kabul was a member of the Afghan armed forces and he was "immediately killed" by fellow soldiers, it says.
The injured US soldier is stable.
This is being seen as the latest so-called "green-on-blue" attack, in which members of the Afghan forces have killed US or coalition soldiers.
No group has so far claimed responsibility for Saturday's attack.
Last month, US Gen Scott Miller escaped unhurt when a bodyguard opened fire on a group of US and Afghan officials in the southern Kandahar province.
The local police chief was killed in that attack, which was later claimed by the Taliban.
In August, a suicide bomber killed three Czech Nato soldiers in eastern Afghanistan. Taliban militants said they carried out the attack.
US Defence Secretary James Mattis has recently pledged to step up training of Afghan troops to prevent further similar attacks.
What US forces are currently in Afghanistan?
American combat operations against the Taliban officially ended in 2014, but more than 8,000 US special forces troops remained in the country to provide training and assistance to Afghan troops.
Last year, US President Donald Trump signalled he would keep US boots on the ground indefinitely amid concerns that the Taliban was gaining ground.
The US forces are part of the Nato-led Resolute Support Mission in Afghanistan, which has more than 16,000 personnel.