The South Korean military said it had mixed up fighter jets after detecting about 180 North Korean warplanes flying more than four hours north of the military border on Friday.
The North Korean plane flew north of the so-called tactical measurement line, drawn up to 12 miles north of the military demarcation line (MDL), the South Korean military said in a statement.
South Korea has mixed up 80 aircraft in response, including F-35A stealth fighters. About 240 aircraft that participated in the Vigilant Storm air exercises with the United States are continuing the exercises, the military said.
A flight of 10 North Korean fighter jets performed similar maneuvers last month, forcing South Korea to scramble fighter jets.
The maneuvers came after North Korea fired more than 80 artillery shots into the sea overnight, and the launch of multiple missiles into the sea on Thursday, including a possible failed intercontinental ballistic missile (ICBM).
The launches prompted the United States and South Korea to expand air exercises that have infuriated Pyongyang.