A North Korean fighter jet is seen taking off from an airbase near Pyongyang in this frame grab from a video released by North Korea's Korean Central Television in 2020 April. Yonhap

North Korea is conducting "major maintenance and expansion" at its Sunchon Airbase, home to the most modern aircraft in the country's aging fleet, according to satellite imagery analyzed by Washington-based think tank the Center for Strategic and International Studies.

Improvements to the base, located about 25 miles northeast of Pyongyang, include repairs and the extension of runways, taxiways and aircraft apron, according to an online report published Thursday by the Beyond Parallel project of CSIS.

The upgrades appear to be timed to coincide with the annual summer training cycle of the Korean People's Air and Air-Defense Force, in which "KPAF aircraft are redeployed for flight training and to test operational readiness," the report's authors, Joseph Bermudez and Victor Cha, wrote.

The airbase is home to Unit 1017 of the KPAF, which operates the country's only MiG-29 and Su-25 aircraft ― Soviet-era fighter planes originally delivered in the 1980s, which remain the most advanced combat planes in North Korea's inventory.

The maintenance and expansion project at Sunchon was first observed in April, when the Su-25 ground attack and MiG-29 fighter aircraft were moved to different bases, which the report said indicated that a surprising number of the planes were still in working order.

The move was "highly suggestive of a far higher level of flight readiness than is normally attributed to these units, although this does not necessarily equate to combat readiness," the report said.

In a blog posting earlier this week, Joseph Dempsey of the London-based International Institute for Strategic Studies analyzed satellite imagery of Sunchon from March and counted 12 MiG-29s and 31 Su-25s at the base before they were moved.

"Given that the IISS previously estimated a total inventory of 34 Su-25s and at least 18 MiG-29s, this means the KPAF was able to fly out nearly all of its [Su-25] fleet, as well as two-thirds of its MiG-29s," Dempsey wrote.

North Korea's "air force has shown ingenuity and adaptability in keeping many obsolescent or obsolete types in service far beyond what might have been expected," Dempsey added.

However, Pyongyang's aging aircraft remain decades behind those of its neighbor in Seoul. South Korea's inventory currently includes 24 F-35A stealth fighters purchased from the United States, with plans to increase the number to 60. The South also unveiled the prototype of its first homegrown fighter aircraft, the KF-21 Boramae, in April.

North Korea has loudly complained about the stealth fighter sale from the United States in the past, calling it a grave provocation that exacerbates tensions on the Korean Peninsula.

While Washington officials have publicly expressed a willingness to restart diplomatic efforts with North Korea, Pyongyang rebuffed the calls this week, with foreign minister Ri Son Gwon saying Wednesday that North Korea was "not considering even the possibility of any contact with the U.S." (UPI)