South Korea scrambled fighter jets and attack helicopters and fired warning shots after North Korean drones violated its airspace, the South's Yonhap news agency reported.
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 10.25am (1.25am Irish time), the military said.
"Several North Korean unmanned aerial vehicles invaded our airspace" in the border areas around Gyeonggi province, Seoul's Joint Chiefs of Staff said in a statement.
The drones are the first confirmed to have come from the isolated neighbour since 2017, when a North Korean drone believed to be on a spy mission crashed and was found on a mountain near the border.
In 2014, a North Korean drone was also discovered on a South Korean border island.
At least one drone came down farther and flew over South Korea's capital Seoul in the latest intrusion, News1 agency reported, citing an unnamed military official.
The military said it tried to shoot down the drones, but it was not clear if it was successful, and whether any of them were armed.
While scrambling to counter the drones, a South Korean KA-1 light attack aircraft crashed shortly after departing its Wonjubase in the country's east, a defence ministry official said.
Its two pilots were able to escape before the crash and are now in the hospital.
South Korea's transport ministry said flights departing from its Incheon and Gimpo airports were suspended following a request from the military.
The suspension began at 1.08pm (4.08am Irish time) at Gimpo and at 1.22pm (4.22am Irish time) at Incheon and lasted for about an hour before flight departures resumed at around 2.10pm (5.10am Irish time), a ministry official told Reuters.
Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said the incident marked the first time that South Korean flights were suspended over "the appearance of North Korean drones", adding they were likely for spying purposes.
"Considering North Korea's poor level of drone development, there is little possibility that they carry the drone attack capabilities used in modern warfare," he added.
"It is speculated that they came over to our area as part of reconnaissance training during the recent winter training."
Drone operations
The incursion is the first time in five years that North Korean drones have invaded South Korean airspace and comes as Pyongyang has carried out an unprecedented blitz of weapons tests this year, including the launch of its most powerful intercontinental ballistic missile yet.
Last week, Pyongyang also fired two short-range ballistic missiles and claimed to have developed new capabilities to take images from space, saying it would be ready to launch a reconnaissance satellite by April next year.
The North's drone operations are a growing security concern in Seoul, but Pyongyang has denied any involvement and accused South Korea of fabricating evidence.
In 2017, Seoul's military fired warning shots at a flying object that entered the country's airspace from North Korea across the Demilitarised Zone.
A year earlier, South Korean soldiers fired warning shots at a suspected North Korean drone that crossed the western part of the border, the most sensitive part of the Demilitarised Zone.
In September 2015, South Korea triggered an anti-aircraft warning and sent an attack helicopter and fighter jet to track down a drone that crossed the border, without success.
In 2014, a South Korean fisherman found the wreckage of a North Korean drone in his net near a frontline island south of the rivals' disputed Yellow Sea border.
