North Korea's neighbors lined up on Wednesday to condemn Pyongyang's claimed hydrogen bomb test, saying it posed a grave threat to regional security.
Several governments promised a firm response as tensions soared again in northeast Asia, many calling for further action by the United Nations against the hermit nation, which is already subject to heavy international sanctions.
"The nuclear test that was carried out by North Korea is a serious threat to the safety of our nation and we absolutely cannot tolerate this," Japanese Prime Minister Shinzo Abe said in Tokyo.
"This clearly violates UN Security Council resolutions and is a grave challenge against international efforts for non-proliferation," he said, adding his country would seek to coordinate efforts among UN members to deal with the action.
South Korean President Park Geun-Hye called the test a "grave provocation" at an emergency meeting of the Country's National Security Council (NSC) convened immediately after the news broke.
"The test is not only a grave provocation to our national security but also a threat to our future... and a strong challenge to international peace and stability," she said, calling for strong sanctions on Pyongyang.
In an earlier statement, Seoul said it would "take all necessary measures including additional sanctions by the UN Security Council... so that the North will pay the price for the nuclear test".
In Washington, the White House would not confirm the test, but vowed to "respond appropriately to any and all North Korean provocations".
There was no immediate response from the Chinese government, but in a commentary the official Xinhua news service said the move was "highly regrettable" and breached UN resolutions.
Using the North's official name, it added: "Turning Northeast Asia into a keg of powder benefits none in the neighborhood, even the DPRK itself, which has pledged to promote economic development."
But it said that Pyongyang's defiance of the international community was "deeply rooted in its sense of insecurity after years of hostility with the United States".
Fellow UN Security Council permanent member France joined the chorus of nations slamming Pyongyang's testing of a nuclear bomb, labelling the move an "unacceptable violation" of UN resolutions and calling for a "strong reaction from the international community".
Australian foreign minister Julie Bishop said her country "condemns in the strongest possible terms" the test, which "confirms North Korea's status as a rogue state and a continuing threat to international peace and security", adding that Canberra would express its concerns to Pyongyang directly and call for stronger UN sanctions.
The test, which came just two days before leader Kim Jong-Un's birthday, was initially detected by international seismology monitors as a 5.1-magnitude tremor next to the North's main Punggye-ri nuclear test site in the northeast of the country.
This picture taken from North Korean TV and released by South Korean news agency Yonhap on January 6, 2016 shows a message of North Korean leader Kim Jong-Un for its first hydrogen bomb test. The Korean letters read that "Let's begin the year of 2016 -- a glorious and victorious year when the historic seventh conference of the Workers' Party of Korea will be held -- with a thrilling sound of the first hydrogen bomb explosion, so that the whole world will look up to our socialist, nuclear-armed republic of Juche and the great Workers' Party of Korea! - Dec. 15, 2015, Kim Jong-Un." Photo: AFP/North Korean TV via YONHAP
Last month, North Korean leader Kim Jong-Un suggested Pyongyang had already developed a hydrogen bomb.
The claim was questioned by international experts and there was continued skepticism over Wednesday's test announcement.
Whether an H-bomb or not, it was North Korea's fourth nuclear test and marked a striking act of defiance in the face of warnings that Pyongyang would pay a steep price if it continued pursuing its atomic weapons program.
The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions. Their failure to prevent a fourth detonation will see calls for more drastic Security Council action this time around.