South Korean military officials say North Korea has flown several drones across their mutual border.
The "unmanned aerial vehicles" violated South Korean airspace in the border areas around Gyeonggi province, said the country's joint chiefs of staff.
The South Korean military said it had fired warning shots before sending up jets and helicopters to shoot them down.
One of the warplanes, a KA-1 light attack aircraft, later crashed, but its two pilots escaped unhurt.
The plane fell to the ground in Hoengseong County, east of Seoul, soon after take-off from an air base in the nearby city of Wonju, according to the air force, quoted by Yonhap news agency.
South Korea also suspended planes taking off and landing at its Incheon and Gimpo airports for about an hour.
The last time a North Korean drone crossed the border was five years ago in June 2017.