North Korea said Wednesday it had successfully conducted a fourth nuclear bomb test, leading to fury from its regional neighbours and condemnation from around the world.
Pyongyang said the device was its first hydrogen bomb, the most powerful type of nuclear weapon, but there was scepticism about the claim from military officials outside North Korea.
"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," White House National Security spokesman Ned Price said.
"We will not accept North Korea as a nuclear state," he said in a statement.
South Korea and Japan threatened increased sanctions against Kim Jong Un's regime in light of the test.
Chinese Foreign Ministry spokeswoman Hua Chunying called on 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."
"It was a breach of UN resolutions and a blow to the Korean Peninsula denuclearization process," the report said.
British Foreign Secretary Philip Hammond, who is in Beijing, said after talks with China's top diplomat Yang Jiechi that the two countries had "agreed to work with other members of the UN Security Council towards a robust international response."
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.
There was scepticism about the hydrogen bomb claim from military officials in China and South Korea.
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.
A South Korean military official said the North Korean blast was too weak to be from a hydrogen bomb, according to Yonhap.
"It is hard to regard this test as that of a hydrogen bomb," the unnamed official was quoted as saying.
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 possible sign 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.
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 North said in an English-language statement.
North Korean leader Kim Jong Un ordered the test on December 15, and signed the final written order on Sunday, according to Xinhua, citing KCNA.
The UN Security Council scheduled a meeting for Wednesday morning in New York to discuss North Korea's surprise announcement.
South Korea's government called a crisis meeting and Japan's security council was also scheduled to meet in response to the test.