South Korea tracked the drones crossing from North Korea over what is known as the Military Demarcation Line between the two countries after detecting them in the skies of the western city of Gimpo at around 10:25 a.m. (0125 GMT), the military said.
The border intrusion by North Korean drones is a clear act of provocation, a South Korean military official said. He did not say if any drones had been shot down but the Yonhap news agency later said South Korea's military fired about 100 shots but failed to shoot any down. As part of its response, the South Korea military also sent surveillance aircraft into the North to photograph its military installations, the official added.
South Korea's transport ministry said earlier that flights departing from its Incheon and Gimpo airports were suspended for about an hour following a request from the military.
While scrambling to counter the drones, a South Korean KA-1 light attack aircraft crashed shortly after departing its Wonju base in the country's east, a defence ministry official said. Its two pilots were able to escape before the crash and are in the hospital.