North Korea said Wednesday it had successfully conducted a test on its first hydrogen bomb, leading to fury and condemnation from around the world even as military experts doubted the validity of the claim.
Pyongyang's regional neighbours were quick to condemn the move, which amounted to North Korea's fourth nuclear bomb test. South Korea and Japan threatened increased sanctions against Kim Jong Un's regime for the act.
"North Korea's provocation is in clear violation of Security Council resolutions and a serious challenge to international peace and security," South Korea's Vice Foreign Minister Lim Sung Nam said, according to Yonhap News Agency.
"South Korea will cooperate with regional partners to make North Korea pay a price," a government statement said.
If confirmed, the test would be a step up in North Korea's nuclear weapons capability and a major setback for efforts by world powers to persuade Pyongyang to cancel its nuclear programme.
Japanese Prime Minister Shinzo Abe said Wednesday's test was "a threat to our country's security and it is totally intolerable."
"While we cannot confirm these claims at this time, we condemn any violation of UN Security Council resolutions," said White House National Security spokesman Ned Price.
NATO Secretary General Jens Stoltenberg said the test "undermines regional and international security," calling it a "clear breach" of UN Security Council resolutions while criticizing North Korea's "inflammatory and threatening rhetoric" in a statement.
Echoing Stoltenberg's call for the communist regime to abandon its nuclear weapons programme, Chinese Foreign Ministry spokeswoman Hua Chunying urged Pyongyang to return to the negotiating table and to give up its nuclear weapons.
The official Xinhua news agency described the test as "highly regrettable" and "a blow to the Korean Peninsula denuclearization process."
EU foreign policy chief Federica Mogherini condemned North Korea's test as a "grave violation" of the country's nuclear obligations, and called it a "threat to the peace and security" of north-east Asia.
Despite the wave of comments condemning the test, it remained unclear whether North Korea had successfully tested a hydrogen bomb.
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.
Randy Bell, the chief data analyst of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBT), said seismic stations had picked up shocks measuring 4.9 on the Richter scale 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.
South Korea's government has called a crisis meeting, although experts from that country have also expressed scepticism about the hydrogen bomb claim.
"It is hard to regard this test as that of a hydrogen bomb," an unnamed official was quoted as saying.
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.
The UN Security Council has scheduled a meeting for Wednesday morning in New York to discuss North Korea's surprise announcement.