North Korea says it has successfully conducted a test of a miniaturised hydrogen nuclear device, marking a significant advance in the isolated state's strike capabilities and raising alarm bells in Japan and South Korea.
The test, the fourth time North Korea has exploded a nuclear device, was ordered by young leader Kim Jong Un, state media said on Wednesday morning.
"The first H-bomb test was successfully conducted at 10am (1300 AEDT) on Wednesday," North Korea's official KCNA news agency said.
In December, Kim appeared to claim his country had developed a hydrogen bomb, also known as a thermonuclear device, a step up from the less powerful atomic bomb, but the United States and outside experts were sceptical at the time.
Some analysts questioned whether Wednesday's test was indeed of a hydrogen device.
North Korea says it has conducted a successful hydrogen bomb test. (AAP) Source: AAP
"North Korea has made claims about its nuclear and missile programs in the past that simply have not held up to investigation," said Melissa Hanham, a Senior Research Associate at the California-based James Martin Center for Nonproliferation Studies.
Yang Uk, a senior research fellow at the Korea Defence and Security Forum, added: "Given the scale it is hard to believe this is a real hydrogen bomb. They could have tested some middle stage kind (of device) between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim."
The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 49km from the Punggye-ri site where the North has conducted nuclear tests in the past.
North Korea's last test, of an atomic device in 2013, also registered at 5.1 on the USGS scale.
The claim of miniaturising, which would allow the device to be adapted as a weapon and placed on a missile, would pose a new threat to the United States and its regional allies, Japan and South Korea.
North Korea has been under UN Security Council sanctions since it first tested an atomic device in 2006 and could face additional measures. The Security Council will meet later on Wednesday to discuss what steps it could take, diplomats said.
The White House said it could not confirm North Korea's claims, but added the United States would respond appropriately to provocations and defend its allies.
Prime Minister Shinzo Abe said Japan would make a firm response to North Korea's challenge against nuclear non-proliferation, calling its test a threat to Japan's security.
South Korea said it would take all possible measures, including possible United Nations sanctions, to ensure Pyongyang paid the price after its fourth nuclear test.
The location of the North Korean earthquake, believed to have been caused by an explosion. Source: USGS
"As of 4.45 this afternoon, no radiation has been detected at any Japanese monitoring posts," chief cabinet secretary Yoshihide Suga told a news conference.
He added that Japan could propose to the United Nations various economic and other sanctions against North Korea in cooperation with the United States, but did not give details.
The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 49km from the Punggye-ri site where the North has conducted nuclear tests in the past.
"We suspect a man-made earthquake and are analysing the scale and epicentre of the quake," a Korea Meteorological Administration official told Reuters by phone.
While the USGS put the depth of the earthquake at 10km, the South Korean agency said it was near the surface. The earthquake was detected just after 10am Seoul time.
The head of an international body set up to monitor a ban on nuclear testing condemned North Korea's fourth nuclear test on Wednesday.
"This act constitutes a breach of the universally accepted norm against nuclear testing," Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organisation which monitors a ban on nuclear tests told Reuters.
"It is also a grave threat to international peace and security," he said.
North Korea plans a major announcement later on Wednesday, South Korean media said.
South Korea's presidential office convened an emergency security meeting while Japan's chief government spokesman said the earthquake was likely caused by a nuclear test.
The last North Korean nuclear test, in 2013, registered at 5.1 on the USGS scale.
North Korea is known to have conducted three nuclear tests and is under UN sanctions for its nuclear and missile programs.
It is not yet known if North Korea has successfully miniaturised a nuclear device small enough to be used as a warhead on a ballistic missile, but the likelihood of the isolated country successfully miniaturising a device increases with each test.
South Korean stocks and the won currency fell slightly after reports of the likely test on Wednesday, and foreign exchange authorities were suspected by dealers to have intervened.
Nuclear test condemned
"Australia condemns in the strongest possible terms the provocative and dangerous behaviour of the North Korean regime," she said.
"Today’s nuclear test confirms North Korea’s status as a rogue state & continuing threat to international peace and security."
Ms Bishop said Australia would make its concerns known directly to the North Korean government and would work to strengthen saction aimed at reducing funding to North Korea's weapons of mass destruction programs.
"North Korea's action fly in the face of international non-proliferation norms, and challenege the Nuclear Non-Proliferation Treaty and the Comprehensive Nuclear-Test-Ban Treaty," she said.
"We call on international bodies, including the UN Security Council, to provide a strong response to North Korea's actions."
South Korea said it would take all possible measures, including possible United Nations sanctions, to ensure Pyongyang paid the price after its fourth nuclear test.
"Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the U.N. resolutions," Cho Tae-yong, a senior security official at the South Korean presidential office said.
President Park Geun-hye (3rd from L) presides over a meeting of the National Security Council at the presidential office of Cheong Wa Dae in Seoul. (AAP) Source: AAP
The White House has said it cannot confirm North Korea's claims, but it would continue to monitor the situation.
"While we cannot confirm these claims at this time, we condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," White House National Security Council spokesman Ned Price said in a statement late on Tuesday.
Mr Price said the United States will continue to "protect and defend our allies in the region," and will "respond appropriately to any and all North Korean provocations."
Japan has strongly protested the nuclear test and says it will be in close contact with the governments of the United States, South Korea, China and Russia over the issue, its top government spokesman said on Monday.
"This is something we can not accept, we strongly protest this," chief cabinet secretary Yoshihide Suga told a news conference.
British Foreign Secretary Philip Hammond said on Wednesday a test of a hydrogen nuclear bomb by North Korea would be a "provocation which I condemn without reservation" and a "grave breach" of U.N. Security Council resolutions.
Hammond tweeted the statement during his two-day trip to Beijing: "If North Korean H-bomb test reports are true, it is a grave breach of #UNSC resolutions and a provocation which I condemn without reservation."
Doubt over North Korea's nuclea capabilities
There has long been scepticism by Washington and nuclear experts on past North Korean claims about H-bombs, which are much more powerful, and much more difficult to make, than atomic bombs.
But a confirmed test would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations. It would also further worsen already abysmal relations between Pyongyang and its neighbours.
North Korean nuclear tests are worrying 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 step for the North. In a hydrogen bomb, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.
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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".
"Thermonuclear weapons are tricky; making one work requires a bit of test experience," he wrote on the North Korea-focused 38 North website.
"The North has now had a nuclear weapons program for more than 20 years. This program has yielded three nuclear tests," he added.
"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."
