North Korea announced Saturday the successful test-firing of a submarine-based ballistic missile -- a technology that would offer the nuclear-armed state a survivable second-strike nuclear capability.

North Korean leader Kim Jong-Un, who personally oversaw the test, hailed the newly developed missile as a "world-level strategic weapon," according to a report by the official KCNA news agency.

There was no immediate independent confirmation of the test, which would mark a major breakthrough for the North's missile programme and violate UN resolutions prohibiting Pyongyang from conducting ballistic missile tests.

Development of a submarine-launched missile capability would take the North Korean nuclear threat to a new level, allowing deployment far beyond the Korean peninsula.

Satellite images earlier this year had shown the conning tower of a new North Korean submarine, which US analysts said appeared to house one or two vertical launch tubes for either ballistic or cruise missiles.

According to the KCNA report, the test was carried out by a sub that dived to launch depth on the sounding of a combat alarm.

"After a while, the ballistic missile soared into the sky from underwater," the agency said, adding that the weapon had been developed on the personal initiative of Kim Jong-Un.

It gave no detail of the size or range of the missile, nor did it specify when the test was carried out.

Kim described the test as an "eye-opening success" on a par with North Korea's successful launch of a satellite into orbit in 2012.

The satellite launch was condemned by the international community as a disguised ballistic missile test and resulted in a tightening of UN sanctions.

- 'World-level' weapon -

Kim said the underwater test meant the Korean military now possessed a "world-level strategic weapon capable of striking and wiping out in any waters the hostile forces infringing upon (North Korea's) sovereignty and dignity."

The announcement of the test came a day after the Korean People's Army (KPA) warned that it was prepared to fire on sight, without warning, at South Korean naval vessels it accused of violating their disputed Yellow Sea border.

While there is no doubt that the North has been running an active ballistic missile development programme, expert opinion is split on just how much progress it has made.

The North has yet to conduct a test showing it has mastered the re-entry technology required for an effective intercontinental ballistic missile.

There are also competing opinions on whether the North has the ability to miniaturise a nuclear device that would fit onto a delivery missile.

North Korea's small submarine fleet is comprised of largely obsolete Soviet-era and modified Chinese vessels, but suggestions that it was experimenting with a marine-based missile system have been around for a while.

The South Korean Defence Ministry cited intelligence reports last September that Pyongyang was understood to be developing a vertical missile launch tube for submarine use.

- Adapted submarine -

Ministry officials said the North's 3,000-ton Golf-class submarine could be modified to fire medium-range ballistic missiles.

And in October last year, a separate satellite image analysis by the US-Korea Institute at Johns Hopkins University identified a new missile test stand at the Sinpo South Shipyard in northeastern North Korea.

The size and design of the stand suggested it was intended to explore the possibility of launching ballistic missiles from submarines or a surface naval vessel, the institute said.

While submarines carrying ballistic missiles could provide the North Korea with a survivable second-strike nuclear capability, the institute had suggested that Pyongyang was likely "years" from achieving the required technology.