Three American contractors killed in an apparent "insider attack" at Kabul airport were helping on maintenance of a small turboprop plane used by the Afghan air force, US defense officials said Friday.

The fatal shootings on Thursday at the heavily guarded airport were carried out by an Afghan wearing military uniform who fired a few bursts from an automatic weapon before he was killed in return fire, officials said. A fourth US contractor was wounded.

The contractors were overseeing maintenance work on a Pilatus PC-12 aircraft, a single-engine plane that is also used by the US Air Force for special operations forces.

An American firm, Sierra Nevada Corp, received a contract in 2012 to provide 18 modified versions of the plane for the Afghan army's special forces.

The attack is "a tragic and grim reminder that Afghanistan still remains a dangerous place in many ways," Pentagon spokesman Rear Admiral John Kirby told reporters, adding the incident was under investigation.

The Americans killed were employed by a private firm, Praetorian Standard, which handles security and logistics contracts for the US Defense Department.

The Taliban on Friday claimed responsibility for the attack.

Western troops and civilians training Afghan forces have faced lethal assaults from Afghans in uniform who turn their guns on their counterparts. NATO troops have adopted special security measures in recent years to try to counter the threat.