Three American contractors were killed and a fourth was wounded by an Afghan solider at the military airport in the capital Kabul, an Afghan air force official told Reuters on Thursday.

"It is unclear yet why he shot these advisers and no one else was there to tell us the reason," the official said, asking not to be named because he was not authorised to give statements to the media. "An investigation has been opened."

The international force in Afghanistan confirmed the shooting took place on Thursday evening.