North Korea's claim of a successful hydrogen bomb test has been met with condemnation from around the world.
South Korea said on Wednesday (local time) 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 United States has also expressed doubt over the validity of the claim, with the White House saying it cannot confirm North Korea's claims that it has successfully conducted a test of a miniaturised hydrogen nuclear device, but it will continue to monitor the situation.
"While we cannot confirm these claims at this time, we condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," White House National Security Council spokesman Ned Price said in a statement late on Tuesday.
Price said the United States will continue to "protect and defend our allies in the region," and will "respond appropriately to any and all North Korean provocations."
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 cannot accept, we strongly protest this," chief cabinet secretary Yoshihide Suga told a news conference.
New Zealand has joined the nations strongly condemning the nuclear test.
"New Zealand views North Korea's actions as highly provocative and irresponsible," Duty Minister Sam Lotu-Iiga said.
“The New Zealand Government strongly urges North Korea to cease its provocative behaviour and commit to not developing, testing or possessing nuclear weapons.”
Mr Lotu-Iiga said New Zealand will be working with other members of the UN Security Council in response to the test.
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.
3 News / Reuters