SEOUL/WASHINGTON, Jan 6 (Reuters)The underground explosion shook the earth so hard that it registered as a seismic event with U.S. earthquake monitors. It put pressure on China to rein in neighboring North Korea.

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.

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 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 U.N. 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.

North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006 and could face additional measures.

The 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.

The 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.

H-BOMB OR NOT?

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.

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 in 2013.

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.

Jeb Bush, whose brother George W. Bush was president when North Korea conducted its first nuclear test in 2006, said on Twitter: “North Korean nuke test shows danger of continuing feckless Obama/Clinton foreign policy.” North Korea has carried out three nuclear tests since.

Clinton, America’s top diplomat from 2009 to 2013, 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.

DOUBTS RAISED

The device tested had a yield of about 6 kilotonnes, according to the office of a South Korean lawmaker on the parliamentary intelligence committee – roughly the same size as the North’s last test, which was equivalent to 6-7 kilotonnes of TNT.

“Given the scale, it is hard to believe this is a real hydrogen bomb,” said Yang Uk, a senior research fellow at the Korea Defence and Security Forum.

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.”

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.

The Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna said preliminary data indicated that the magnitude of the seismic event detected in North Korea was lower than a similar one caused by a North Korean nuclear test in 2013. DM

Photo: North Korean leader Kim Jong-Un holds up his ballot during the fifth session of the 12th Supreme People’s Assembly of North Korea at the Mansudae Assembly Hall in Pyongyang April 13, 2012, in this picture released by the North’s KCNA on April 14, 2012. REUTERS/KCNA