Doubts raised about North Korea's 'H-bomb of justice' claims

A large crowd celebrated in front of Pyongyang's main train station as the announcement was read on a big video screen, with people taking videos or photos of the screen on their mobile phones and applauding and cheering.

But South Korea's spy agency thought the estimated explosive yield from the explosion was much smaller than what even a failed H-bomb detonation would produce.

North Korea's state media stood firm in saying the test was a self-defence measure against a potential US attack.

"The (country's) access to H-bomb of justice, standing against the US, the chieftain of aggression..., is the legitimate right of a sovereign state for self-defence and a very just step no one can slander," it was stated.

In Seoul and elsewhere there was high-level worry. South Korean president Park Geun-hye ordered her military to bolster its combined defence posture with US forces.

She called the test a "grave provocation" and "an act that threatens our lives and future."

Japanese prime minister Shinzo Abe said: "We absolutely cannot allow this."

Washington and nuclear experts have been sceptical about past North Korean claims about H-bombs, which are much more powerful and much more difficult to make than atomic bombs.

A confirmed test would further worsen already abysmal relations between Pyongyang and its neighbours and lead to a strong push for tougher sanctions on North Korea at the United Nations. The Security Council quickly announced an emergency meeting.

Nato Secretary-General Jens Stoltenberg said the nuclear weapons test is a "clear breach" of UN Security Council resolutions and "undermines regional and international security".

Mr Stoltenberg said in a statement Wednesday: "I condemn the continued development by North Korea of nuclear weapons and ballistic missile programs and its inflammatory and threatening rhetoric."

He called on North Korea's government to "fully respect its international obligations and commitments".

A successful H-bomb test would be a big advance. Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.

A South Korean lawmaker said the country's spy agency told him in a private briefing that Pyongyang may not have conducted an H-bomb test given the relatively small size of the seismic wave reported.

An estimated explosive yield of 6.0 kilotons and a quake with a magnitude of 4.8 were detected, lawmaker Lee Cheol Woo said the National Intelligence Service (NIS) told him. That is smaller than the estimated explosive yield of 7.9 kilotons and 4.9-magnitude quake reported after the 2013 nuclear test, he said, and only a fraction of the hundreds of kilotons that a successful H-bomb test's explosion would usually yield.

Even a failed H-bomb detonation typically yields tens of kilotons, the NIS told Mr Lee, who sits on the parliament's intelligence committee.

"I'm pretty sceptical," said Melissa Hanham, senior researcher at the James Martin Centre for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, California. "The seismic data indicates it would be very small for a hydrogen test.

"It seems just too soon to have this big technical achievement," she said.

While also noting the quake was likely too small for an H-bomb test, Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University, said the North could have experimented with a "boosted" hybrid bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel.

It could take weeks before the true nature of the test is confirmed by outside experts - if they are able to do so at all.

US Air Force aircraft designed to detect the evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a US base on the Japanese island of Okinawa. Japanese media said Tokyo mobilised its own reconnaissance aircraft for sorties over the Sea of Japan to try to collect atmospheric data.

Ferenc Dalnoki-Veress, a physicist, scientist-in-residence and professor at the James Martin Center, said it may not be possible for the monitors to ever determine if Wednesday's explosion was caused by a hydrogen bomb.

"For that, you might need to have the particulates," he said. "But maybe we'll be lucky."

The test was unexpected in part because North Korea's last nuclear test was nearly three years ago and Kim Jong Un did not mention nuclear weapons in his annual New Year's speech.

Some outside analysts had speculated Mr Kim was worried about deteriorating ties with China, the North's last major ally, which has shown greater frustration at provocations and a possible willingness to allow stronger UN sanctions.

Chinese foreign ministry spokeswoman Hua Chunying told reporters that Beijing "firmly opposes" Pyongyang's purported test and is monitoring the environment on its border with North Korea near the test site.

Some analysts say the North has not likely achieved the technology needed to manufacture a miniaturised warhead that could fit on a long-range missile capable of hitting the US mainland.