Three American contractors were killed in an apparent insider attack in the Afghan capital Kabul, an Afghan air force official told Reuters news agency.

"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 also 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 US Major General Harold Greene, who became the most senior American military official to die in action overseas since the war in Vietnam.

The incident on Thursday followed a spate of attacks by the armed group Taliban across the country. 

Earlier on Thursday, a suicide bomb attack on a funeral in east Laghman province killing 16 people including 12 civilians and four policemen.

Separately on Wednesday, at least 11 Afghan policemen were killed and several others wounded in an attack on a checkpoint in the Andar District of Ghazni province.

The new international "Resolute Support" mission involving a small contingent of around 12,000 mostly US troops started on January 1 and is to focus on training Afghanistan's national security force.

Source: Reuters