SEOUL (Reuters) - North Korea said on Sunday it had conducted a submarine-launched ballistic missile test supervised by leader Kim Jong Un and it was a “great success” that gave the country “one more means for powerful nuclear attack.”

North Korea fired one missile from a submarine off its east coast on Saturday, South Korea’s military said, amid concerns that the isolated state might conduct a nuclear test or a missile launch ahead of a rare ruling party meeting in May.

The missile flew for about 30 km (18 miles), a South Korean Defense Ministry official said late on Saturday, adding its military was trying to determine whether the launch may have been a failure for unspecified reasons.

But the North’s official news agency KCNA said an underwater test-fire of a ballistic missile was “another great success,” without disclosing the date and place of the event which was guided by leader Kim.

“It fully confirmed and reinforced the reliability of the Korean-style underwater launching system and perfectly met all technical requirements for carrying out ... underwater attack operation,” KCNA said.

“The successful test-fire would help remarkably bolster the underwater operational capability of the KPA navy, he said, adding that it is now capable of hitting the heads of the South Korean puppet forces and the U.S. imperialists anytime as it pleases,” it said, quoting Kim. KPA refers to the North’s military.

The U.S. Strategic Command said on Saturday 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 on Saturday called on the European Union to unilaterally adopt additional sanctions on North Korea if the missile launch was confirmed.

North Korea 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, a series of test launches were believed to have been failures, and its state media carried footage that appeared to have been edited to fake success, according to experts who have seen the visuals.

North Korea is banned from nuclear tests and activities that use ballistic missile technology under U.N. sanctions dating to 2006 and most recently adopted in March but it has pushed ahead with work to miniaturize a nuclear warhead and develop an intercontinental ballistic missile (ICBM).

North Korea will hold a congress of its ruling Workers’ Party in early May for the first time in 36 years, at which leader Kim is expected to formally declare the country is a strong military power and a nuclear state.

North Korea’s Foreign Minister Ri Su Yong told the Associated Press in New York on Saturday that his country is ready to halt nuclear tests if the United States suspends its annual military exercises with South Korea. North Korea made a similar demand in January after its fourth nuclear test.

Asked if the United States would consider a halt, Katina Adams, a spokeswoman for the State Department’s East Asia bureau, said the exercises demonstrate the U.S. commitment to the alliance with South Korea and enhance “combat readiness.”

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 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 ICBM putting the mainland United States within range. It is seen to be several years from acquiring the technology, according to arms experts.