Americans killed in Afghan insider attack

Three Americans killed in an 'insider attack' in Kabul were helping on maintenance of a plane used by the Afghan air force, US defence officials say.

The fatal shootings on Thursday at the city's 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 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.

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

The Taliban has claimed responsibility for the attack.