By Ju-min Park and Tony Munroe
North Korea says it has successfully tested a miniaturised hydrogen nuclear bomb, prompting scepticism among experts and officials who doubt Pyongyang has achieved such a major advance in its strike capability.
The test, the fourth time the isolated state has exploded a nuclear device, was ordered by leader Kim Jong Un and successfully conducted at 10am on Wednesday (local time), North Korea's official KCNA news agency said.
"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.
South Korean intelligence officials and several analysts questioned whether Wednesday's explosion was a test of a full-fledged hydrogen device, pointing to the fact that it was roughly as powerful as North Korea's last atomic test in 2013.
But the development unnerved South Korea and Japan and drew international criticism, including from China and Russia, North Korea's two main allies.
United Nations Secretary-General Ban Ki-moon condemned North Korea's action, calling it "profoundly destabilising for regional security", while US House Speaker Paul Ryan said it "looks like a provocation".
No countries were given advance warning of a nuclear test, South Korea's intelligence service said, according to lawmakers briefed by intelligence officials.
In previous such tests, Pyongyang had notified China, Russia and the United States beforehand, they said.
While a fourth nuclear test had been long expected, the claim that it was a hydrogen device, much more powerful than an atomic bomb, came as a surprise, as did the timing.
It ensures that North Korea will be a key topic during the US presidential campaign.
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 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 not a true fusion bomb capable of the massive multi-megaton yields these bombs produce".
The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 49 km 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.