SEOUL (REUTERS) - North Korea fired what may be a submarine-launched ballistic missile (SLBM) on Wednesday (Oct 2), which would be the first test in three years of what had been a relatively young but rapidly progressing programme to deliver nuclear weapons.

The launch comes hours after the North announced it would resume nuclear talks with the United States this weekend, potentially ending a months-long deadlock that followed a vow by North Korea leader Kim Jong Un and US President Donald Trump to make progress.

The exact type of the missile and the launch platform remain unclear, but it appears to be a step that "pushes the envelope", said Mr Joshua Pollack, a leading expert on nuclear and missile proliferation and editor of Nonproliferation Review journal.

WHAT HAPPENED?

A missile was launched from the sea soon after 7am on Wednesday about 17km north-east of the coastal city of Wonsan, the site of one of North Korea's military bases used for previous missile launches.

Japan initially said two missiles were launched but later clarified it was likely one projectile that went through stage separation. The projectile hit the waters in Japan's Exclusive Economic Zone (EEZ), the Japanese government said.

South Korea's Defence Minister Jeong Kyeong-doo said an Aegis destroyer detected one missile launch, which flew 450km in a lofted trajectory 910km high.

North Korea's state news agency KCNA said on Thursday the missile was a Pukgukson-3 test-fired at a high angle in the waters off Wonsan designed to contain external threat and bolster self-defence, but gave no other details.

Photos released in the North's official Rodong Sinmun newspaper showed a black-and-white painted missile emerging and clearing the surface of the water, then the booster rocket igniting to propel it into the sky.

A US official, speaking on condition of anonymity, said that, according to initial intelligence, the missile was a submarine-capable ballistic missile launched from a platform at sea.

WHAT NEW THREAT DOES IT POTENTIALLY REPRESENT?

If the missile had been launched on a standard trajectory, the range would have been up to 1,900km, which would put it in the medium-range missile class.

That missile would have all of South Korea and Japan within range. Three analysts said the Pukguksong-3 appeared to be a new design that has enhanced range and stability compared with a version tested in 2016.

It was probably launched from a test platform and not a submarine, which would be the final stage of testing, said Dr Kim Dong-yub, a military expert at Kyungnam University's Institute of Far Eastern Studies in Seoul.

A launch from a submarine deployed in the surrounding waters would pose greater difficulty for missile defences.

The threat of a submarine-launched ballistic missile (SLBM) grows exponentially with the range of the submarine.

The North's existing Romeo-class submarines, which were built in the 1990s, are believed to have a range of about 7,000km, potentially making a one-way trip to near Hawaii possible.

But they are diesel-electric powered and very noisy, making them highly vulnerable to detection, especially by US forces with their decades of experience tracking Soviet submarines.

WHAT IS THE PROGRESS OF NORTH KOREA'S SLBM PROGRAMME?

North Korea began testing submarine-launched ballistic missiles in 2015 and conducted four submarine launches by August 2016, when a two-stage solid-fuel Pukguksong missile flew about 500km on a lofted trajectory. That test was considered a success.

There has been no known tests since then to suggest the North has made progress in developing an SLBM of intermediate or long ranges.

Those previous launches were conducted near the port city of Sinpo, about 110km from Wonsan and home to many of the North's fleet of submarines, believed to be one of the world's largest.

Despite the size of the fleet, most of the vessels are believed to be small or vintage Soviet-era models and only one is believed to be an experimental submarine capable of carrying a ballistic missile.

North Korea said in July leader Kim Jong Un inspected a large, newly built submarine that was near operational deployment.

Analysts said photos released on the North's state media suggested the vessel could be a modified Romeo class type with an enlarged hull, not the larger submarine satellite images have suggested was being built at the Sinpo shipyard.

SECOND-STRIKE CAPABILITY?

Submarine-launched ballistic missiles are considered key to delivering a second-strike capability that can be used to retaliate against a nuclear attack.

To be assured of the capability, the submarine must not only have the ability to launch a nuclear ballistic missile but also the endurance to sail within range of the enemy.

Military analysts are sceptical the North's submarine programme has reached the level of technical sophistication to achieve a second-strike capability.

OTHER RECENT MISSILE TESTS

North Korea has conducted nine launches since leader Kim met Mr Trump at the Demilitarised Zone inter-Korean border on June 30 and pledged to resume nuclear talks.

All but the one on Wednesday has been short-range missiles and rockets that would be a fast and effective way to attack South Korea and US forces stationed there.

Before Mr Kim entered an unofficial moratorium on missile and nuclear tests to engage in dialogue with Mr Trump, the North test-fired an intercontinental ballistic missile in November 2017.

If launched on a standard trajectory, that missile would have had a range of up to 13,000km, putting the mainland United States in strike distance.

But experts are doubtful the North has mastered the technology to build a nuclear warhead small enough to be mounted on a missile that can withstand re-entry to the atmosphere and to guide it with precision to hit the target.