South Korean military officials say that North Korea has flown five drones across their mutual border.
The country’s Joint Chiefs of Staff said “unmanned aerial vehicles” violated South Korea’s airspace in border areas around Gyeonggi province.
One drone flew to the northern edge of the capital Seoul before returning across the border.
Jets and attack helicopters were deployed but 100 rounds fired from the helicopters failed to kill them.
A South Korean military official said they had since lost track of all the drones, but they were no longer in flight.
The incursion comes amid heightened tensions on the Korean peninsula, with the North carrying out a record number of missile tests this year.
The drones were seen crossing the border at around 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 survived.
The plane crashed in Hongseong County, east of Seoul, shortly after taking off from an air base in the nearby city of Wonju, the air force quoted Yonhap as saying.
South Korea also suspended take-offs and landings 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.
“Our military will continue to respond vigorously and firmly to North Korea’s provocations,” the military added.
In response, South Korea’s military said it also sent surveillance planes to the North to photograph its military installations.
The last time a North Korean drone crossed the border was five years ago in June 2017, when tensions between the two Koreas were last at their highest.
North Korean drones pose a security threat to South Korea because they can be used to spy on and attack South Korea.
Earlier this month, the North claimed it had conducted major tests needed to help develop its first spy satellite, which could be used to monitor South Korea. It released an aerial photo of Seoul, which it said was taken during the test.
Experts believe South Korea is working to improve and improve its weapons, while pressuring the United States to ease sanctions in any future talks.
