South Korea's military said it scrambled fighter jets after detecting about 180 North Korean military flights north of the two countries' border over four hours on Friday.
The North Korean aircraft flew north of the so-called tactical action line, north of the Military Demarcation Line between the two Koreas, South Korea's military said in a statement. The virtual line is used as a basis for South Korean air defence operations, a South Korean official said.
South Korea scrambled 80 aircraft, including, F-35A stealth fighters, in response. About 240 aircraft participating in the Vigilant Storm exercises with the US continued the drills, the military said.
A flight of 10 North Korean warplanes made similar manoeuvres last month, prompting South Korea to scramble jets.
The manoeuvres came after North Korea fired more than 80 rounds of artillery 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 extend air drills that have angered Pyongyang.