North Korea's H-bomb announcement met with skepticism, condemnation

US said early analysis of underground activity "is not consistent" with the North's claim of a successful H-bomb test

Soon after the ground shook around its nuclear testing facility, North Korea trumpeted its first hydrogen bomb test, a self-proclaimed "H-bomb of justice" that would mark a major and unanticipated advance for its still-limited nuclear arsenal.

The announcement today by the defiant, impoverished country was met with widespread skepticism, as well as a strong condemnation by the UN Security Council, which said it would begin work on a resolution for new international sanctions.

North Korea's fourth nuclear test likely pushed its scientists and engineers closer to their goal of building a warhead small enough to place on a missile that can reach the US mainland.

But South Korea's spy agency thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce, and the White House said its early analysis of underground activity "is not consistent" with the North's claim of a successful H-bomb test.

There was a burst of jubilation and pride in Pyongyang. A North Korean TV anchor said the test of a "miniaturized" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level."

A large crowd celebrated in front of the capital's main train station as the announcement was read on a big video screen, with people applauding, cheering and recording the report on their mobile phones.

North Korea's state media called the test a self-defence measure against a potential US attack.

"The (country's) access to H-bomb of justice, standing against the US, the chieftain of aggression ..., is the legitimate right of a sovereign state for self-defense and a very just step no one can slander."

There was high-level concern in Seoul and elsewhere. South Korean President Park Geun-hye ordered her military to bolster its combined defense posture with US forces. She called the test a "grave provocation" and "an act that threatens our lives and future." Japanese Prime Minister Shinzo Abe said, "We absolutely cannot allow this."

US Defence Secretary Ash Carter spoke by phone with his South Korean counterpart Han Min-Koo, and they agreed that a North Korean nuclear test would be an "unacceptable and irresponsible provocation," according to Carter's spokesman, Peter Cook.

Cook said Carter reaffirmed the US Treaty commitment to defend South Korea, which he said includes "all aspects of the United States' extended deterrence", an allusion to a longstanding US promise to defend South Korea with nuclear weapons if necessary.

In saying an early analysis by the US was "not consistent with the claims that the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest added that nothing has happened in the last 24 hours to change Washington's assessment of Pyongyang's technical or military capabilities.

The US is still doing the work needed to learn more about the North's test, he added.