The UN Security Council was considering action against North Korea after the reclusive regime said Wednesday it had successfully conducted a test on its first hydrogen bomb.
The claim prompted international outcry even as experts doubted its validity.
"This act is profoundly destabilizing for regional security and seriously undermines international non-proliferation efforts," UN Secretary General Ban Ki-moon said. "I condemn it unequivocally."
The council was holding an emergency meeting in New York to discuss further measures against North Korea, which is already under a number of economic and commercial sanctions imposed by the Security Council for previous nuclear tests.
Pyongyang's regional neighbours were quick to condemn the move, which amounted to North Korea's fourth nuclear bomb test.
Motohide Yoshikawa, Japanese ambassador to the UN and a member of the council, said that the international community needs to respond to the test "with the most robust language that we can find."
"There are measures that we have agreed in that past that, if there is another nuclear test, there should be further action by the Security Council," Yoshikawa 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."
Chinese Foreign Ministry spokeswoman Hua Chunying urged Pyongyang to return to the negotiating table and to give up its nuclear weapons.
NATO Secretary General Jens Stoltenberg said the test "undermines regional and international security," criticizing North Korea's "inflammatory and threatening rhetoric" in a statement.
Despite the wave of comments condemning the act, 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 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.
Wednesday, January 6, 2016 - 20:50
Wednesday, January 6, 2016 - 14:28