North Korea has confirmed it fired a submarine ballistic missile, in what analysts say is the most provocative test by the hermit kingdom since it started nuclear talks with the United States in 2018.

Key points: Analysts say the latest test "pushes the envelope"

Analysts say the latest test "pushes the envelope" It is unclear if the missile was launched from a submarine or a platform at sea

It is unclear if the missile was launched from a submarine or a platform at sea SLBMs can be used to retaliate against nuclear attack

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

Wednesday's test is the first in three years of what has been a relatively young but rapidly progressing programme to deliver nuclear weapons.

Analysts say it is likely a reminder by Pyongyang of the weapons capability it has been aggressively developing as it gears up for the new round of talks.

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.

The missile was launched from the sea soon after 7am on Wednesday (local time), about 17 kilometres 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 split into two.

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The projectile hit the waters in Japan's Exclusive Economic Zone, 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.

South Korea expressed concern and Japanese Prime Minister Shinzo Abe condemned the launch, saying it was a violation of UN Security Council resolutions.

North Korea rejects UN Security Council resolutions that ban Pyongyang from using ballistic missile technology, saying they are an infringement of its right to self-defence.

It is unclear if the missile was launched from a submarine or a platform at sea.

CNN, citing a US official, said the missile was launched from an underwater platform, which North Korea has previously done at the early stage of the SLBM program in 2015.

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. 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 7,000km, 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.

North Korea began testing the submarine ballistic missile program (SLBM) 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 further 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 Mr Kim inspected a large, newly built submarine and that its operational deployment was near.

People watch a TV showing a file image of North Korean leader Kim Jong Un during a news program at the Seoul Railway Station, South Korea. ( AP: Ahn Young-joon )

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 that 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

Talks aimed at dismantling North Korea's nuclear and missile programs have been stalled since a second summit between Mr Trump and Mr Kim in Vietnam in February ended without a deal.

President Donald Trump meets North Korean leader Kim Jong Un at the DMZ in June. ( AP: Susan Walsh )

The two leaders then met at the Demilitarized Zone border between the two Koreas in June and pledged to reopen working-level talks within weeks.

Since then, North Korea has conducted nine launches.

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 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 (ICBM) in November 2017.

If launched on a standard trajectory, that missile would have had a range of up to 13,000km, putting mainland US 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.

David Wright, missile expert with the Union of Concerned Scientists, put the range of the missile tested on Wednesday at about 1,900km at standard trajectory.

Hours before Wednesday's launch, North Korean Vice Foreign Minister Choe Son Hui said in a statement the working-level talks with the US would be held on Saturday — a development that could potentially break what had been months of stalemate.

North Korea's previous missile launch was on September 10, also hours after Ms Choe had expressed Pyongyang's willingness for talks with the US.

Leif-Eric Easley, a professor at Ewha University in Seoul, said: "North Korea tends to raise the stakes before negotiations in an effort to win unearned concessions."

Vipin Narang, a nuclear expert at the Massachusetts Institute of Technology, said the timing of the latest launch enhanced leverage for the North and also signalled Pyongyang was in for the long haul in its talks with Washington.

"The risk is that testing such a system causes the US to walk away before this weekend, but Mr Kim probably bet that the US is so invested in the talks taking place and making progress … that the US won't walk away."

Mr Trump has played down North Korea's recent series of short-range launches, saying in September the US and North Korea "didn't have an agreement on short-range missiles" and that many countries test such weapons.

However, analyst Moon Seong Mook at the Seoul-based Korea Research Institute for National Strategy said Mr Trump would find it difficult to say again everyone can conduct short-range missile tests.

"A submarine-launched ballistic missile is a very dangerous weapon," he said.

AP/Reuters