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.
"It is unclear yet why he shot these advisers and 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.
A rise in so-called "insider attacks" in Afghanistan has eroded trust between Afghan and international troops in the final years of the combat mission that ended in 2014, prompting foreign forces to scale back interaction with their allies.
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 around 12,000 mostly U.S. troops started on January 1 and is to focus on training Afghanistan's national security force.
