The Air Force said Friday it has retrieved the black box of an F-15K fighter jet that crashed into a mountain earlier this week.

The flight data recorder is expected to help in finding out the exact cause of the incident that killed two pilots in their 20s.

The F-15K aircraft crashed into Mount Yoohak in Chilgok, North Gyeongsang Province, Thursday afternoon on its way back to Daegu Air Base following aerial training.

The two pilots, identified as Capt. Choi and 1st Lt. Park, apparently failed to eject from the plane. Debris from the plane and body parts were found at the site.

An X-ray analysis showed that the (collected) body parts match those of the two," an Air Force official told reporters.

He pointed out it's still early to talk about the exact cause of the crash and added that there was "no problem with weather conditions" for the flight mission, although they were using "instrument flight" not visual flight.

The Air Force has decided to posthumously promote the two by one rank. It plans to hold a funeral for them at the Daegu base, home to the 11th Fighter Wing, on Saturday at the consent of the bereaved families.

The remains will be buried at the Daejon National Cemetery in a ceremony to be attended by Defense Minister Song Young-moo, the Air Force chief Gen. Lee Wang-keun and other dignitaries.

The Air Force introduced Boeing's F-15K Slam Eagle, the plane involved in the crash, in 2008. The crashed jet had flown a total of 2,158 hours.

Choi, who was in the front seat, had more than 890 flight hours and Park had flown the aircraft for around 280 hours, according to the official. Choi's wife is reportedly an Air Force officer as well.

Shortly after the incident, the Air Force suspended the flight of all warplanes except for those needed for national defense.

"Whether to resume flights of the F-15Ks will be decided as a probe into the crash proceeds, and other aircraft will soon be put back into operation," the official said.

South Korea has around 60 F-15Ks, a key element of its air power. (Yonhap)