The "unidentified projectile" fired toward the East Sea is presumably a long-range ballistic missile, Seoul and Tokyo said, amid fears that Pyongyang could be preparing to test-fire an ICBM.
Pyongyang has fired a suspected ballistic missile, apparently continuing a weapons testing blitz that could culminate with a major, long-range missile launch.
South Korea's military said on Thursday that it had detected "an unidentified projectile launched into the East Sea" without offering further detail.
The Yonhap news agency reported that it was a "suspected long-range ballistic missile", citing Seoul's Joint Chiefs of Staff.
Japan's coastguard issued an emergency warning to vessels over a "potentially ballistic missile possibly launched from North Korea".
Pyongyang extending its barrage of weapons tests has raised concerns that the country is preparing for a flight of its biggest-yet intercontinental ballistic missile (ICBM).
READ MORE: US: North Korea testing new ICBM system
Suspected ICBM launch
South Korea and the United States have recently warned that Pyongyang appears to be preparing to test-fire an Intercontinental Ballistic Missile (ICBM) at full range for the first time since 2017.
Such testing was paused while leader Kim Jong-un embarked on high-profile negotiations with then-US president Donald Trump, but the talks collapsed in 2019 and diplomacy has been stalled since.
Despite biting international sanctions over its weapons programs, Pyongyang has doubled-down on Kim's drive to modernise its military, and last week conducted a failed test of what analysts suspect was a new, long-range ICBM.
From hypersonic to medium-range ballistic missiles, the North test-fired a string of weaponry in January.
It also recently twice tested what it claimed were components of a "reconnaissance satellite", although Seoul and Washington said it was a new intercontinental ballistic missile system.
Washington said the tests were a "serious escalation" and would be punished with fresh sanctions, warning that North Korea appeared to be preparing for a long-range ICBM test, possibly disguised as a space launch.
Last week, North Korea test-fired what Seoul said was most likely a ballistic missile, although the launch ended in failure, exploding mid-air in the skies above the capital.
The North's state media has stayed mum on that launch but analysts have suggested it was of Pyongyang's so-called "monster missile", the Hwasong 17, a new ICBM system that had never been launched before.
North Korea will mark the 110th anniversary of the birth of founder Kim Il Sung on April 15, and analysts predict Pyongyang will conduct an ICBM or satellite launch as part of the celebrations.