The Taliban have claimed responsibility for an apparent "insider attack" at Kabul airport in which three American contractors and an Afghan were killed.
Details of the Thursday evening shooting are still unclear, with a spokesman for NATO's Resolute Support mission saying the incident is under investigation.
A US defence official in Washington told AFP that the American victims, who were employed under a US Defense Department contract to help train the Afghan air force, died from gunshot wounds.
Taliban spokesman Zabihullah Mujahid said a member of the insurgent movement was responsible for the attack.
"A brave Afghan mujahid infiltrator working in the military side of Kabul airport opened fire on invading American soldiers, killing three Americans," he said in a statement sent to media.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Western troops and civilians training Afghan security forces have faced lethal assaults from Afghans in uniform who turn their guns on their counterparts.
NATO troops have adopted special security measures in recent years to try to counter the threat.
The airport in the Afghan capital is heavily guarded, with one section devoted to commercial aircraft and another area set aside for a NATO contingent.
Thursday's attack came after a surge in Taliban violence over the past year.
At least nine people were killed earlier Thursday in the country's east when a suicide bomber struck at a funeral for victims of a roadside bomb attack.
Most NATO combat troops pulled out of Afghanistan last year but a small contingent of about 12,000 remain in the country, including roughly 10,600 American forces.
The American soldiers, along with other NATO troops and private contractors, are helping the Afghans improve their logistics and build up a fledgling air force.
