North Korea has fired a submarine-launched ballistic missile off its east coast, amid concerns the isolated state might conduct a nuclear test or a missile launch ahead of a ruling party meeting in May.
The North fired the missile to the northeast on Saturday night, South Korea's office of the Joint Chiefs of Staff said.
North Korea will hold a congress of its ruling Workers' Party in early May for the first time in 36 years, at which its leader Kim Jong Un is expected to say the country is a strong military power and a nuclear state.
The missile flew for about 30km, a South Korean Defence Ministry official said, adding its military was trying to determine whether the launch may have been a failure for unspecified reasons.
South Korea's Yonhap news agency said the missile flew "for a few minutes," citing a government source.
The US Strategic Command said it had detected and tracked a North Korean submarine missile launch but it did not pose a threat to North America.
State Department spokesman John Kirby said launches using ballistic missile technology were "a clear violation of multiple UN Security Council resolutions."
France called on the European Union to unilaterally adopt additional sanctions on North Korea if the missile launch was confirmed.
The European Union in March expanded trade and financial sanctions on North Korea, following up on harsh new measures imposed by the UN Security Council.
The North first attempted a launch of the submarine-based missile last year and was seen to be in the early stages of developing such a weapons system, which could pose a new threat to its neighbours and the United States if it is perfected.
However, follow-up test launches were believed to have fallen short of the North's expectations as its state media footage appeared to have been edited to fake success, according to experts who have seen the visuals.
South Korea's military has said it is on high alert over the possibility that the isolated North could conduct its fifth nuclear test "at any time" in defiance of UN sanctions after setting off what it said was a hydrogen device in January.
Satellite images show North Korea may have resumed tunnel excavation at its main nuclear test site, similar to activity seen before the January test, a US North Korea monitoring website reported last week.
South Korea and the United States believe the North is working to develop a submarine-launched ballistic missile system and an intercontinental ballistic missile (ICBM) putting the mainland United States within range.
North Korea is banned from nuclear tests and activities that use ballistic missile technology under UN sanctions dating to 2006 and most recently adopted in March but it has pushed ahead with work to miniaturise a nuclear warhead and develop an ICBM.