KABUL - An Afghan soldier killed three US 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 authorised 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. A US defence official in Washington told AFP that the American victims, who were employed under a US Defence 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.

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 US 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 US troops started on Jan 1 and is to focus on training Afghanistan’s national security force.  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.