South Korean military officials claim North Korea has flown several drones across their shared border.
The “unmanned aerial vehicles” violated South Korean airspace in border areas around Gyeonggi province, according to 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 fired warning shots before sending up jets and helicopters to shoot them down.
However, the attempt to bring down the drones had so far failed and was continuing, it added.
The drones were spotted crossing the border around 10:30 a.m. 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 unharmed.
According to the air force, which was quoted by Yonhap, the plane crashed in Hoengseong County, east of Seoul, shortly after taking off from an air base in nearby Wonju.
South Korea also halted planes taking off and landing at its Incheon and Gimpo airports for about an hour.
The latest incursion, according to a South Korean military official, was a clear act of provocation by Pyongyang.
“Our military will continue to respond thoroughly and resolutely to North Korea’s provocations,” the military added.
The last time a North Korean drone crossed the border was in June 2017, when tensions between the two Koreas were at their highest.
These drones are seen as a threat to South Korea’s security, as they could be used both to spy on it and launch an attack.