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.
One of the drones flew to the north of the capital, Seoul.
The South Korean military said it had fired warning shots before sending up jets and helicopters to shoot them down.
However, the attempt to bring down the drones had failed so far and was continuing, it added.
The incursion comes amid increased tensions on the Korean peninsula, with the North conducting a record number of missile tests this year.
The drones were spotted crossing the border at about 10:30 local time (01:30 GMT).
One of the South Korean warplanes involved in the operation, 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.
South Korea also suspended planes taking off and landing at its Incheon and Gimpo airports for about an hour.
A South Korean military official described the latest incursion as a clear act of provocation by Pyongyang.
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