KABUL- An Afghan soldier killed three U.S. contractors and wounded a fourth yesterday 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 on condition of anonymity.

"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.