North Korea's claim of a successful hydrogen bomb test has been met with condemnation from around the world.
South Korea said on Wednesday it would take all possible measures, including possible United Nations sanctions, to ensure Pyongyang pays the price after its fourth nuclear test.
"Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the UN resolutions," Cho Tae-yong, a senior security official at the South Korean presidential office said.
However South Korea's intelligence agency said the device may not have been a hydrogen nuclear bomb, South Korea's Yonhap News Agency reported.
South Korea's meteorological agency said separately that it had not detected any radiation after the test.
The secretive North, under UN sanctions for its nuclear and missile programs, has so far conducted three nuclear tests - in 2006, 2009 and 2013 - all at Punggye-ri, near the location of Wednesday's earthquake of 5.1 magnitude.
The head of an international body set up to monitor a planned ban on nuclear testing condemned the test and called it a "wake-up call" for the international community.
"This act constitutes a breach of the universally accepted norm against nuclear testing," Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO).
"It is also a grave threat to international peace and security.
"I sincerely hope that this will serve as the final wake-up call to the international community to outlaw all nuclear testing by bringing the CTBT into force."
Japan strongly protests the test and will be in close contact with the governments of the United States, South Korea, China and Russia over the issue, its top government spokesman said.
"This is something we can not accept, we strongly protest this," chief cabinet secretary Yoshihide Suga told a news conference.
China's Xinhua state news agency said the test runs counter to the goal of denuclearisation and warned that any practice that disrupts stability in northeast Asia is "undesirable and unwise".
British Foreign Secretary Philip Hammond said North Korea's actions are a "provocation which I condemn without reservation" and a "grave breach" of UN Security Council resolutions.
Germany said it would summon North Korea's ambassador on Wednesday, foreign ministry spokesman Martin Schaefer told a government news conference, adding the step was "a strong signal, even a protest".
NATO's chief says North Korea must end its nuclear weapons program, adding the US-led alliance's voice to international condemnation.
"North Korea should abandon nuclear weapons and existing nuclear and ballistic missile programs in a complete, verifiable and irreversible manner and engage in credible and authentic talks on denuclearisation," NATO Secretary-General Jens Stoltenberg said in a statement.
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