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.

Related stories

Experts doubt claims of North Korean hydrogen bomb test

North Korea claims successful hydrogen bomb test

Worldwide anger over North Korean nuclear test

How important is North Korea's latest nuclear test?

South Korea urges China to join sanctions on North Korea

Latest news

Syrian opposition rules out future role for President al-Assad

The Syrian opposition said Friday it would not accept any role for President Bashar al-Assad in the future of the war-torn country, reacting to a recent US shift saying that removing al-Assad is no longer a priority for Washington.

Russian Army integrates breakaway forces of Georgian province

Parts of the small fighting forces of the Georgian breakaway province of South Ossetia have been placed under Russian military control, Russian Defence Minister Sergei Shoigu said on Friday.

Czech Republic's Pilsner Urquell beer is now Japanese

Japanese brewing company Asahi completed its takeover of the Czech brewery Pilsner Urquell on Friday, Asahi said in a statement.

Judge approves 25-million-dollar settlement of Trump University case

A US district judge on Friday approved a 25-million-dollar settlement of lawsuits and state fraud allegations against Trump University, the US president's now-defunct business venture.

Former Thai premier Thaksin to junta on reconciliation: 'Cut me out'

Former Thai prime minister Thaksin Shinawatra on Friday announced that he is not interested in the junta-led reconciliation process, three days after the junta handed him a half-a-billion-dollar tax bill for his past business deal.

Dalic: We welcome possible deal between Agrokor and banks

The government welcomes the possibility of an agreement being concluded between the Agrokor food company and creditor banks, and the bill on vitally important companies is not a fallback plan but the result of the government's care for the overall economic and financial stability of Croatia, Deputy Prime Minister and Minister of Economy Martina Dalic told a press conference in Zagreb on Friday.

Croatia, China sign action plan for cooperation in agriculture

The Croatian and Chinese ministries of agriculture on Friday signed an action plan for cooperation in the field of agriculture for the period 2017-2018, the Croatian ministry said in a statement.

ZSE indices up, Agrokor shares in focus of investor interest

The Zagreb Stock Exchange (ZSE) indices on Friday rose by more than 1.8%, with stocks of the Agrokor food and retail concern being in the focus of investor interest again.

Berlin police defend handling of Berlin market attacker

Berlin police defended themselves on Friday against accusations that they stopped surveillance on Berlin Christmas market attacker despite knowing in June 2016 he was dangerous.

Croatia, creditors tailor emergency measures to save tottering giant

Croatia's tottering retail and food giant Agrokor reached an agreement with its creditors, putting its debts standby and allowing it to continue working during emergency restructuring, the Croatian branch of Austria's Erste Bank said Friday.

Agrokor's creditors say standstill agreement to go into force today

A standstill agreement regarding the Agrokor concern's existing financial obligations to banks will take effect on Friday, additional capital will be injected into the concern in the coming days and the concern will be actively restructured, which includes a change of its management, it was said on Friday after a meeting between Agrokor's suppliers and creditor banks.

Palestinians, UN slam Israel's new settlement plan

Palestinians, Israeli activists and the UN lambasted the government of Israeli Prime Minister Benjamin Netanyahu on Friday, a day after it gave the go-ahead for the first new West Bank settlement in a quarter of a century.

South Sudan rebels release three abducted foreign oil workers

South Sudanese rebels have released three foreign engineers they abducted in early March in the oil-rich Upper Nile region, Foreign Affairs Ministry official Mawein Makol Arik said on Friday.

Turkish opposition: Imprisoned party chief has gone on hunger strike

The head of Turkey's pro-Kurdish opposition party has launched a hunger strike from prison.

European leagues threaten Champions League schedule clashes

The European Professional Football Leagues (EPFL) on Friday threatened schedule clashes on Champions League matchdays in an ongoing dispute with the governing body UEFA.

Danish court revokes citizenship of IS volunteer

A Danish appellate court on Friday stripped a man of his Danish citizenship for volunteering to fight for the extremist Islamic State in Syria.

Banks and Agrokor agree on key elements of standstill agreement

Member banks of the coordinating committee of financial creditors and representatives of the Agrokor food company have in principle agreed on key elements of a standstill agreement, which is expected to be signed later today, announcing changes in the company's management team, Erste Bank said in a statement on Friday afternoon.

Syrian man on trial in Sweden; mosque attack labelled terrorism

A Syrian man went on trial Friday in the southern Swedish city of Malmo, charged with terrorism and arson after an attack last year on a building used as an assembly hall by Shiite Muslims.