Seoul - South Korea fired warning shots Tuesday at a flying object that entered the country's airspace from North Korea across the Demilitarized Zone, Seoul's military said.
The object had not been identified and its radar course was being investigated, the military said in a statement.
One official told AFP it appeared to be a drone.
The official gave no details, but previous drones flown across the border have been similar in size to hobby craft.
The military did not indicate whether the object had been shot down, or had gone back over the border, one of the most heavily-fortified locations in the world.
"The military is keeping tight surveillance over the North and maintaining high vigilance," the statement said.
The South's Yonhap news agency said around 90 rounds were fired from a machine-gun at the object.
Tensions are high in the region over the nuclear-armed North's weapons ambitions, and the incident comes only two days after its latest missile launch.
That saw a medium-range Pukguksong-2, which Pyongyang said used solid fuel, travel around 500 kilometres.
The North has carried out two atomic tests and dozens of rocket firings since the beginning of last year in its quest to develop a missile capable of delivering a nuclear warhead to the continental United States.
The administration of US President Donald Trump has said military action was an option under consideration, sending fears of conflict spiralling.
South Korea has repeatedly accused the North of flying suspected spy drones across the tense border.
As recently as January last year, 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 demilitarized 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.
A year earlier, 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.
A few months previously, crashed drones equipped with cameras were found in three different places near the border.
After a joint investigation with US experts, Seoul said the recovered drones were flown from North Korea and had been pre-programmed to fly over South Korean military installations.
North Korea denied any involvement and accused the South of "fabricating" evidence.