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 leader Kim Jong Un and 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.

WHAT HAPPENED?

A missile was launched from the sea soon after 7 a.m. on Wednesday (2200 GMT Tuesday) about 17 km 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.

It is unclear if the missile was launched from a submarine or 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 their missile defence.

The threat of a submarine-launched ballistic missile 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 an 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?