North Korea says successfully conducts first H-bomb test

North Korea said it successfully tested a miniaturised hydrogen nuclear device on Jan. 6, claiming a significant advance in the isolated state's strike capability and setting off 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. 

"The first H-bomb test was successfully conducted at 10:00 (0130 GMT) on Wednesday," North Korea's official KCNA news agency said. 

Last month, 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. 

South Korean intelligence officials and several analysts questioned whether the Jan. 6 explosion was indeed a full-fledged test of a hydrogen device. 

The device 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. 

"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." 

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...