An Afghan soldier killed three U.S. contractors and wounded a fourth on Thursday at Kabul's military airport, an Afghan air force official said.
An increase in so-called insider attacks in Afghanistan eroded trust between government forces and their international allies in the final years of the combat mission that ended in 2014, prompting foreign forces to scale back interaction with Afghans.
The Taliban claimed responsibility for the attack saying a militant infiltrator had carried it out.
The Afghan air force official said it was not clear why the attacker had killed the advisers.
"No one else was there to tell us the reason," the official told Reuters on condition of anonymity because he was not authorized to speak to the media.
"An investigation has been opened."
The international force in Afghanistan confirmed the shooting took place on Thursday evening. It also confirmed that one Afghan had been killed but it did not specify that he was the attacker.
Despite the introduction of new measures, including a requirement that coalition forces be armed at all times and stricter vetting procedures for Afghan security members, incidents continue to occur sporadically.
One of the worst attacks took place last August, killing U.S. Major General Harold Greene, who became the most senior American military official to die in action overseas since the war in Vietnam.
The new international "Resolute Support" mission involving a small contingent of about 12,000 mostly U.S. troops started on Jan. 1 and is to focus on training Afghanistan's national security force.
(Reporting by Mirwais Harooni and Jessica Donati; Editing by Gareth Jones)