North Korea fired a submarine-launched ballistic missile on Saturday off its east coast, South Korea said, amid concerns that 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 from an area off its east coast at about 6:30 p.m. (0930 GMT), the South'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 30 km (18 miles), a South Korean Defence Ministry official said by telephone, 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 U.S. State Department in Washington said it was aware of reports the North had launched what appeared to be a ballistic missile.

"Launches using ballistic missile technology are a clear violation of multiple UN Security Council resolutions," said State Department spokesman John Kirby.

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 neighbors 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 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 U.N. sanctions after setting off what it said was a hydrogen device in January.

Satellite images show that North Korea may have resumed tunnel excavation at its main nuclear test site, similar to activity seen before the January test, a U.S. North Korea monitoring website reported on Wednesday.

South Korea and the United States, as well as experts, 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 U.N. sanctions dating back to 2006 and most recently adopted in March but it has pushed ahead with work to miniaturize a nuclear warhead and develop an ICBM.

A senior U.S. official said this week that North Korea should take a lesson from Iran which has agreed to roll back its nuclear program in an agreement with Western powers in return for lifting of major sanctions but the North has shown no sign of entering into such a pact.