United nations Motohide Yoshikawa.jpg
Photograph: EPA/JUSTIN LANE

The UN Security Council pledged to take "significant measures" against North Korea after the reclusive regime said Wednesday it had successfully conducted a test on its first hydrogen bomb.

The council expressed strong condemnation, and said in a statement it would begin working on a resolution - most likely involving further sanctions - against North Korea, whose nuclear test claim sparked a global outcry even as experts doubted its validity.

The Security Council, which began imposing economic and commercial sanctions on North Korea a decade ago because of its nuclear activity, said that "a clear threat to international peace and security continues to exist" because of the communist state's actions.

After the council's closed-door, emergency meeting, the Japanese ambassador to the UN said the body was united in its decision.

"Our aims is to have swiftly a new resolution which has a robust content," Motohide Yoshikawa said. "We have some further ideas and we will be discussing that with the other members."

Pyongyang's regional neighbours were quick to condemn the move, which amounted to North Korea's fourth nuclear bomb test since 2006.

"South Korea will cooperate with regional partners to make North Korea pay a price," a government statement from Seoul said.

If confirmed, the test would be a step up in North Korea's nuclear weapons capability and a major setback for long-running diplomatic efforts to persuade Pyongyang to end its nuclear programme.

China, a longtime ally of North Korea, also expressed its concern, with the Foreign Ministry urging Pyongyang to return to the negotiating table and to give up its nuclear weapons.

"This act is profoundly destabilizing for regional security and seriously undermines international non-proliferation efforts," UN Secretary General Ban Ki-moon said. 

US Secretary of State John Kerry denounced the test as a "highly provocative act" and called on the country to "end these provocations and choose a better path."

President Barack Obama plans to call Japanese Prime Minister Shinzo Abe and South Korean President Park Geun Hye to express US security support for its regional allies. Kerry and Defence Secretary Ashton Carter have also spoken with their counterparts.

Despite the wave of criticism, experts and governments were sceptical North Korea had actually even tested a hydrogen bomb.

White House spokesman Josh Earnest said that an initial US analysis, based on seismic analysis and data from regional partners, showed that the activity was "not consistent with North Korean claims of a successful hydrogen bomb test."

An international nuclear test watchdog said the shock from the fourth nuclear test was of the same size as a previous one in 2013, indicating that it may not have been a powerful hydrogen weapon that exploded but rather a more conventional nuclear bomb.

Randy Bell, the chief data analyst of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBT), said seismic stations had picked up shocks measuring magnitude-4.9 in both cases.

While Bell refused to talk about the possible nature of the blast, diplomats told dpa in Vienna that the shock was far smaller than what was to be expected from a hydrogen bomb.

"The big hydrogen bombs that have been tested by official nuclear weapons states in the past caused bigger earthquakes," one diplomat said.

A Chinese military expert said data so far "cannot support" the claims of a hydrogen bomb test and that further analysis was required, according to Chinese broadcaster CCTV on Twitter.

North Korea carried out three nuclear weapons tests between 2006 and 2013, followed each time by new and stricter UN Security Council sanctions against Pyongyang.

The first indication of Wednesday's test was a tremor that registered as a small earthquake in the region of Kilju in north-east North Korea, where previous atomic tests have been carried out.

China's earthquake monitoring centre in Beijing said that the shock was of magnitude 4.9. US measurements put the magnitude at 5.1.

According to a North Korean statement, the test at 10:00 am (0130 GMT) showed that North Korea had "joined the advanced ranks of nuclear weapons states" possessing hydrogen bombs.

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