(Updates with N.Korea's KCNA report)
By Jack Kim
SEOUL, Oct 2 (Reuters) - North Korea fired what may be a submarine-launched ballistic missile (SLBM) on Wednesday, 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 U.S. 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 Joshua Pollack, a leading expert on nuclear and missile proliferation and editor of Nonproliferation Review.
A missile was launched from the sea soon after 7 a.m. on Wednesday (2200 GMT Tuesday) about 17 km (11 miles) northeast 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 450 km (280 miles) in a lofted trajectory 910 km (565 miles) 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 U.S. 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 1,900 km (1,200 miles), which would put it in the medium-range missile class.
That missile would have all of South Korea and Japan within range. 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 7,000 km, 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 U.S. 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 500 km (310 miles) 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 110 km 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.
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 Trump at the demilitarised zone (DMZ) 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 fast and effective way to attack South Korea and U.S. forces stationed there.
Before Kim entered an unofficial moratorium on missile and nuclear tests to engage in dialogue with Trump, the North test-fired an intercontinental ballistic missile (ICBM) in November 2017.
If launched on a standard trajectory, that missile would have had a range of up to 13,000 km, 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.
(Reporting by Jack Kim, Josh Smith, Joyce Lee and Sangmi Cha in Seoul and Idrees Ali in Washington; Editing by Raju Gopalakrishnan, Lisa Shumaker and Lincoln Feast.)
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