South Korea scrambled fighter jets after a group of 10 North Korean military aircraft flew close to the border dividing the two Koreas, South Korea's Joint Chiefs of Staff (JCS) said.
It comes amid heightened tensions over repeated North Korean missiles tests.
A JCS statement said the incident happened late on Thursday and early Friday Korean time.
According to security chiefs, the planes flew as close as seven miles north of the inter-Korean border in what is a highly unusual incident.
F-35 jets and other warplanes were scrambled in an immediate response.
It comes just days after North Korea fired another missile over Japan in its seventh projectile test in just a fortnight.
The move, thought to be tied to an upcoming visit to Thailand by US Vice President Kamala Harris, happened days after residents in Japan were told to take shelter amid the first project test in five years.
And the unspecified ballistic missile landed outside Japan's Exclusive Economic Zone (EEZ), according to the South Korean Coast Guard.
The EEZ is Japan's protected border zone stretching from Tokyo Bay to the Izu and Ogasawara Islands.
The launch was made moments ago - early on Sunday in its local time - but South Korea's Joint Chiefs of Staff gave no further details, such as how far the weapon flew.
The Japanese government also says North Korea has fired a possible ballistic missile.
But the launch is North Korea's sixth round of weapons tests in two weeks, and came hours after the United States and South Korea wrapped a new round of naval drills off the Korean Peninsula's east coast.
The drills involved a US aircraft carrier.
North Korea's military warned Saturday that the U.S. redeployment of the aircraft carrier near the Korean Peninsula is causing a "considerably huge negative splash" in regional security.
