North Korea fired a submarine-launched ballistic missile off its east coast on Saturday, 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 at about 6:30 p.m., 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 kilometres, 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. Strategic Command said it had detected and tracked a North Korean submarine missile launch but it did not pose a threat to North America.

'Clear violation'

U.S. State Department spokesman John Kirby said launches using ballistic missile technology were "a clear violation of multiple UN Security Council resolutions."

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.

This file photo shows North Korean leader Kim Jong Un looking through a periscope of a submarine during his inspection of the Korean People's Army (KPA) Naval Unit 167.

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 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 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.

Calls for additional sanctions

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.

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.

Meanwhile, France called on the European Union to "unilaterally adopt additional sanctions" on North Korea.

"We call on the international community to adopt a firm and united reaction so that North Korea stops its provocations," a spokesman for the French Foreign Ministry said in a statement.

Minister wants U.S. to suspend exercises

In an interview with The Associated Press on Saturday, North Korea's foreign minister said his country is ready to halt its nuclear tests if the United States suspends its annual military exercises with South Korea.

He also defended the country's right to maintain a nuclear deterrent and warned that North Korea won't be cowed by international sanctions. For those waiting for the North's regime to collapse, he had this to say: Don't hold your breath.

North Korea on Saturday fired what appeared to be a ballistic missile from a submarine off its northeast coast, South Korean defence officials said. (Lee Jin-man/Associated Press)

Foreign Minister Ri Su Yong, in his first interview with a Western news organization, held firm to Pyongyang's longstanding position that the U.S. drove his country to develop nuclear weapons as an act of self-defence. At the same time, he suggested that suspending the military exercises with Seoul could open the door to talks and reduced tensions.

"If we continue on this path of confrontation, this will lead to very catastrophic results, not only for the two countries but for the whole entire world as well," he said, speaking in Korean through an interpreter.

"It is really crucial for the United States government to withdraw its hostile policy against the DPRK and as an expression of this stop the military exercises, war exercises, in the Korean Peninsula. Then we will respond likewise."

DPRK is an abbreviation for North Korea's official name, the Democratic People's Republic of Korea.

Ri, who spoke calmly and in measured words, a contrast to the often bombastic verbiage used by the North's media, claimed the North's proposal was "very logical."