SEOUL/WASHINGTON, Jan 7 - North Korea said it successfully tested a nuclear bomb on Wednesday, drawing threats of further sanctions even though the United States and weapons experts voiced doubts the device was as advanced as the nation claimed.
The underground explosion triggered U.S. quake monitors and angered China, putting pressure on Beijing to rein in its neighbor and ally. The test also alarmed Japan, whose leader told U.S. President Barack Obama in a phone call that a firm global response was needed, Kyodo news agency said.
The U.N. Security Council said it would work immediately on significant new measures, a threat diplomats said could mean an expansion of sanctions against Pyongyang, although major powers might baulk at an all-out economic offensive.
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 Obama of running a "feckless" foreign policy that enabled North Korea to bolster its nuclear arms capabilities.
U.S. congressional sources said Republican leaders of the U.S. House of Representatives were considering a vote as soon as next week to impose stiffer punishment on foreign companies doing business with Pyongyang.
North Korea has a long history of voicing bellicose rhetoric against the United States and its Asian allies without acting on it, but Pyongyang's assertion 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 U.S. West Coast, South Korea and Japan.
The U.S. 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.
China, the North's main economic and diplomatic backer, said it would lodge a protest with Pyongyang. The explosion also drew criticism from Russia and Japan.
"It is a grave security threat to damage the peace and safety of the region and international society," Japanese broadcaster NHK quoted Prime Minister Shinzo Abe as saying after he spoke to Obama.
'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.
North Korea called the device the "H-bomb of justice", but its state news agency also said Pyongyang would act as a responsible nuclear state and would not to use its nuclear weapons unless its sovereignty was infringed.
The impoverished state boasts of its military might to project strength globally but also plays up the need to defend itself from external threats as a way to maintain control domestically.
Hydrogen bombs use a two-step process of fission and fusion that releases substantially more energy. However, it will likely take several days to determine more precisely what kind of device Pyongyang set off as a variety of sensors, including "sniffer planes", collect evidence.
A U.S. government source said Washington 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. The latest blast also occurred in the same location as earlier tests.
The United States had been anticipating a North Korean nuclear test for some time, with intelligence indicating possible preparations such as evidence of new excavations of underground tunnels at the site.
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 programme.
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 defences.