Korean Air will sell five of its planes to US aerospace firm Sierra Nevada Corporation, the airline said in an exchange filing on Wednesday.

Sierra Nevada recently won a $13 billion US Air Force contract to develop a successor to the E-4B Nightwatch, known as the Doomsday plane due to its ability to survive a nuclear war and act as a command and control centre during emergencies.

Sierra Nevada did not immediately respond to a request for comment.

The Korean Air planes being sold are four-engined Boeing 747-8s, a source familiar with the matter said.

The four current E-4 planes, which have been in service since the 1970s, are modified Boeing 747-200s.

The sale, valued at 918 billion Korean won ($674 million), is in line with Korean Air's medium to long-term plan to dispose of older aircraft and replace them with newer generation ones, the Korean Air filing said.

Korean Air will sell the planes in September 2025, the filing said.