The South Korean Air Force has grounded its fleet of F-5 fighter aircraft, following a fatal accident that killed three pilots.

The F-5 aircraft will remain grounded for safety checks, until the cause of the accident is confirmed, according to The Korea Times.

According to an air force spokesman, the air force has deployed an investigation team to find the cause of the accident, which may be due to the pilots losing their bearings or the two jets colliding with each other.

The air force is planning to introduce 60 F/A-50 light attack aircraft, a modified variant of the T-50 supersonic trainer jet, to replace its aging F-4/5 jets.

The air force currently operates about 250 F-4/5 aircraft, most of which will retire between 2105 and 2018.

The two aircraft, a single-seat F-5E and twin-seat F-5F, crashed into a mountain near Gangneung, Gangwon Province, during a training mission.

A light supersonic fighter aircraft, the F-5 was developed by Northrop in the 1960s.