Twelve North Korean warplanes flew in formation on Thursday, apparently staging air-to-surface firing drills amid tensions on the Korean Peninsula, according to South Korean media.
Around 30 South Korean fighter jets also flew out in response, according to Yonhap News Agency, citing Seoul officials.
It was apparently a tit-for-tat reaction to recent military drills between South Korea and the US, the media outlet reported.
The formation flight of eight fighter jets and four bombers took place north of the inter-Korean air boundary at around 2 p.m. (0500GMT), and they were thought to have conducted air-to-surface firing drills, according to the Joint Chiefs of Staff (JCS).
Some 30 South Korean fighter jets were immediately deployed to the area in an "overwhelming" response to the flight, which is a move unseen over the past year, the media outlet reported, citing the JCS.
The North's activities came after Seoul and Washington recently conducted air, ground, and naval drills in response to a series of North Korean missile launches.