North Korea said it successfully tested a powerful nuclear bomb on Wednesday, drawing threats of further sanctions even though the US and weapons experts voiced doubts the device was as advanced as the isolated nation claimed.

The underground explosion shook the earth so hard that it registered as a seismic event with US earthquake monitors. It put pressure on China to rein in neighbouring North Korea.

The U.N. Security Council said it would begin working immediately on significant new measures in response to North Korea, a threat diplomats said could mean an expansion of sanctions.

North Korea has been under Security Council sanctions since it first tested an atomic device in 2006. After a nuclear test in 2013, the Security Council took about three weeks to agree a resolution that tightened financial restrictions and cracked down on Pyongyang's attempts to ship and receive banned cargo.

In the United States, Republican presidential candidates seized on the test to accuse President Barack Obama of running a "feckless" foreign policy that enabled North Korea to bolster its nuclear arms capabilities.

US congressional sources said Republican leaders of the US House of Representatives were considering a vote as soon as next week to broaden sanctions against North Korea by imposing stiffer punishments on foreign companies doing business with Pyongyang.

While North Korea has a long history of voicing bellicose rhetoric against the United States and its Asian allies without acting on it, the assertion by Pyongyang on Wednesday that it had tested a hydrogen device, much more powerful than an atomic bomb, came as a surprise.

North Korea also said it was capable of miniaturising the H-bomb, in theory allowing it to be placed on a missile and potentially posing a new threat to the US West Coast, South Korea and Japan.

The US State Department confirmed North Korea had conducted a nuclear test but the Obama administration disputed the hydrogen bomb claim.

"The initial analysis is not consistent with the claim the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest told reporters. He said any nuclear test would be a "flagrant violation" of Security Council resolutions.

The explosion drew criticism, including from China and Russia. Beijing, the North's main economic and diplomatic backer, said it will lodge a protest with Pyongyang.

'H-BOMB OF JUSTICE'

Wednesday's nuclear test took place two days ahead of what is believed to be North Korean leader Kim Jong Un's birthday.

"Let the world look up to the strong, self-reliant nuclear-armed state," Kim wrote in what North Korean state TV displayed as a handwritten note.

North Korea called the device the "H-bomb of justice."

While the Kim government boasts of its military might to project strength globally, it also plays up the need to defend itself from external threats as a way to maintain control domestically.

It will likely take several days to determine more precisely what kind of nuclear device Pyongyang set off as a variety of sensors, including "sniffer planes," collect evidence.

Hydrogen bombs pack an explosion that can be more powerful than an atomic bomb as it uses a two-step process of fission and fusion that releases substantially more energy.

A US government source said the United States believes North Korea had set off the latest in a series of tests of old-fashioned atomic bombs of which it has dozens.

The source said the size of the latest explosion was roughly consistent with previous tests believed to have been conducted with A-bombs rather than H-bombs. The latest test occurred in the same geographical location, with the same geological profile, as earlier tests.

The United States had been anticipating a North Korean nuclear test for some time, as intelligence surveillance produced indications of possible preparations, including evidence of new excavations of underground tunnels at the site.

The USGS reported a 5.1 magnitude seismic event that South Korea said was 49 km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.

South Korean intelligence officials and several analysts also questioned whether Wednesday's explosion was a test of a full-fledged hydrogen device, pointing to its having been roughly as powerful as North Korea's last atomic test.

Stocks across the world fell for a fifth consecutive day as the North Korea tension added to a growing list of geopolitical worries and China fueled fears about its economy by allowing the yuan to weaken further.

The Republicans added North Korea to a list of what they assert are Obama's foreign policy failures, including Syria's civil war, the rise of Islamic State and the agreement to curb Iran's nuclear program.

They also blamed his former secretary of state, Hillary Clinton, the Democratic Party front-runner in the race for the November presidential election.

Asked about North Korea, Republican White House hopeful Donald Trump told CNN that "China should solve that problem" or face trade retaliation from the United States. "South Korea should pay us and pay us very substantially for protecting them," he said.

Clinton condemned North Korea's action as a "dangerous and provocative act" and said the United States should respond with more sanctions and stronger missile defenses.

North Korea has long coveted diplomatic recognition from Washington, but sees its nuclear deterrent as crucial to ensuring the survival of its third-generation dictatorship.

The North's state news agency said Pyongyang would act as a responsible nuclear state and vowed not to use its nuclear weapons unless its sovereignty was infringed.

Joe Cirincione, a nuclear expert who is president of Ploughshares Fund, a global security organisation, said North Korea may have mixed a hydrogen isotope in a normal atomic fission bomb.

"Because it is, in fact, hydrogen, they could claim it is a hydrogen bomb," he said. "But it is not a true fusion bomb capable of the massive multi-megaton yields these bombs produce." – Reuters