North Korea said it has conducted a powerful hydrogen bomb test in a defiant and surprising move that, if confirmed, would be a huge jump in Pyongyang's quest to improve its still-limited nuclear arsenal.

A television anchor said the North had tested a "miniaturised" hydrogen bomb, elevating the country's "nuclear might to the next level" and providing it with a weapon to defend against the United States and its other enemies.

The statement said the test was a "perfect success" and the announcement was celebrated on the streets of Pyongyang, where a large crowd gathered in front of the city's main train station to watch the test announcement on a big video screen. Some people took videos or photos of the screen on their mobile phones while others applauded and cheered.

South Korean president Park Geun-hye ordered her military to bolster its combined defence posture with US forces and 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."

China, the North's closest ally, said the reported test was carried out in defiance of the international community and urged North Korea to refrain from acts that might worsen tensions on the Korean Peninsula.

There has long been scepticism from Washington and nuclear experts 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, however, would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations.

The Security Council, which is responsible for international sanctions, quickly announced an emergency meeting on North Korea.

North Korean nuclear tests capture the world's attention because each new blast is seen as pushing North Korea's scientists and engineers closer to their goal of building a bomb small enough to place on a missile that can reach the US mainland.

A successful H-bomb test would be a big new step for the North, and the announcement prompted scepticism. Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic weapons 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.

Writing in December, after North Korean leader Kim Jong Un bragged of H-bomb capabilities, nuclear expert Jeffrey Lewis noted that building such a bomb "would seem to be a bit of a stretch for the North Koreans".

But he wrote on the North Korea-focused 38 North website: "The North has now had a nuclear weapons programme for more than 20 years. This programme has yielded three nuclear tests. North Korean nuclear scientists have access to their counterparts in Pakistan, possibly Iran and maybe a few other places. We should not expect that they will test the same fission device over and over again."

One expert in Seoul said the reported 5.1 magnitude of the quake set off by Wednesday's test was probably too small to be an H-bomb test.

However, the North could have experimented with a "boosted" bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel, said Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University.

A South Korean politician said the country's spy agency told him that Pyongyang may not have conducted an H-bomb test given the relatively small size of the seismic wave reported, according to Yonhap news agency.

North Korea had not conducted a nuclear explosion since early 2013 and Kim did not mention the country's nuclear weapons in his New Year's speech. Some outside analysts speculated that he was worried about deteriorating ties with China, the North's last major ally, which has shown signs of greater frustration at provocations and a possible willingness to allow stronger UN sanctions.

Six-nation negotiations on dismantling North Korea's nuclear programme in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim's father, Kim Jong Il, who died in late 2011.