South Korea has scrambled 30 military aircraft after the North flew 12 warplanes near its border.
It came just hours after Pyongyang fired two ballistic missiles into the sea in its sixth round of missile tests in less than two weeks.
Eight North Korean fighter jets and four bombers flew in formation and were believed to have conducted air-to-surface firing drills, according to South Korea's military.
The military said South Korea responded by scrambling 30 fighter jets and other warplanes, although they did not engage in any clash with the North Korean aircraft.
It refused to provide further details, including how close to the border the North Korean planes flew.
Tensions have risen sharply on the Korean Peninsula as North Korea's recent barrage of missile tests prompted South Korea, the US and Japan to conduct joint drills in response.
Thursday's launch came two days after North Korea fired an intermediate-range missile over Japan for the first time in five years.
Foreign experts said the missile fired on Tuesday involved an intermediate-range weapon capable of reaching the US Pacific territory of Guam and beyond.