A South Korean F-5 fighter jet crashed into a mountain in the central part of the country Thursday, but the pilot ejected safely just before impact, the Korean Air Force said.

The one-seater F-5E took off from an air base in Cheongju, about 137 kilometers south of Seoul, at around 10:48 a.m. for a training exercise, but it crashed into a mountain near Jeungpyeong shortly before noon, the Air Force said in a release.

The 32-year-old Air Force captain, only identified by his surname Lee, ejected from the jet after he was unable to make an emergency landing when the nose of the plane rapidly pointed upwards, it said.

The pilot was safely rescued and sent to a nearby military hospital, officials said.

Military officials and firefighters were dispatched to the scene, but they are barred from entering the site due to risk of additional explosion. An explosion took place 50 minutes after the crash, according to officials. 

Investigations are currently under way to determine the cause of the incident involving the old jet, which has been in operation by the Air Force since 1978.

Since deployment in 1978, the combat jet has flown about 6,610 hours and was scheduled to retire in 2017, according to the Air Force.

Among 200 F-5s operated by the South Korean Air Force since 1970s, all F-5 A and B variants have phased out, while F-5 E and F variants are expected to be out of service in the next decade when they finish a full life cycle set at 38 years.

Seoul has adopted 60 F-15Ks from Boeing since 2002 and is seeking to buy 60 advanced jets to replace the old aircraft in the coming years. The latest crash involving the F-5 came after the state arms procurement agency on Wednesday announced that it is resetting its fighter jet purchasing program after failing to select a replacement aircraft, further delaying the replacement of the aging fleet of F-4s and F-5s.

The plane crash is the latest incident involving different variants of the F-5s. Three F-5s crashed in Gangwon Province in 2010, killing a total of five pilots. (Yonhap News)