North Korea said Wednesday it had conducted a powerful hydrogen bomb test, a move that, if confirmed, would be a huge jump in Pyongyang's quest to improve its still-limited nuclear arsenal.
The North said in a broadcast that the test was successful but monitoring posts in Japan failed to detect radiation.
A television anchor on state TV said in a statement that the North had tested a "miniaturized" hydrogen bomb, elevating the country's "nuclear might to the next level" and providing it with a weapon to defend against the United States and its other enemies.
The statement said the test was a "perfect success." Crowds gathered outside a large video screen near a Pyongyang train station to cheer as the state TV anchor, dressed in a pink traditional Korean hanbok, delivered the statement.
The North's state news agency said the nation will not give up its nuclear program as long as the United States maintained what it called "its stance of aggression."
It also said North Korea will act as a responsible nuclear state and vowed not to use its nuclear weapons unless its sovereignty was infringed. It said it will not transfer its nuclear capabilities to other parties.
South Korea's President Park Geun-hye convened an emergency national security council meeting and vowed to take a tough response to the North's bomb test, saying that the government "must get North Korea to face corresponding measures based on closed cooperation with the international community."
She added: "It's not only grave provocation of our national security, but also an act that threatens our lives and future. It's also a direct challenge to world peace and stability."
Park also ordered the military to bolster its combined defense posture with the U.S. military, saying South Korea will sternly deal with any additional provocation by North Korea.
NATO's chief said on Wednesday that North Korea must end its nuclear weapons program, adding the U.S.-led alliance's voice to international condemnation of Pyongyang.
"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 de-nuclearization," NATO Secretary-General Jens Stoltenberg said in a statement.
There has long been skepticism by Washington and nuclear experts on past North Korean claims about H-bombs, which are much more powerful, and much more difficult to make, than atomic bombs. But a confirmed test would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations. It would also further worsen already abysmal relations between Pyongyang and its neighbors.
Some analysts questioned whether Wednesday's test was indeed of a hydrogen device.
"North Korea has made claims about its nuclear and missile programs in the past that simply have not held up to investigation," said Melissa Hanham, a Senior Research Associate at the California-based James Martin Center for Nonproliferation Studies.
Yang Uk, a senior research fellow at the Korea Defense and Security Forum, added: "Given the scale it is hard to believe this is a real hydrogen bomb. They could have tested some middle stage kind (of device) between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim."
At Japanese monitoring posts, "as of 4.45 this afternoon, no radiation has been detected at any Japanese monitoring posts," chief cabinet secretary Yoshihide Suga told a news conference.
Failed diplomatic efforts
The test comes amid failed diplomatic efforts to persuade the North to give up its nuclear ambitions. Six-nation negotiations on dismantling North Korea's nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim Jong Un's father, Kim Jong Il, who died in late 2011.
National Security Council spokesman Ned Price said the United States is "aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site and have seen Pyongyang's claims of a nuclear test."
He called on North Korea to abide by its international obligations and commitments and said the U.S. consistently made clear that it will not accept North Korea as a nuclear state and will continue to defend U.S. allies in the region.
The head of the U.N. Comprehensive Test Ban Treaty Organization, which monitors worldwide for nuclear testing, said if confirmed, a nuclear test by North Korea would be a breach of the treaty and a grave threat to international peace and security.
North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006 and could face additional measures. The Security Council will meet later on Wednesday to discuss what steps it could take, diplomats said.
Japanese Prime Minister Shinzo Abe said on Wednesday the nation would make a firm response to North Korea's challenge against nuclear non-proliferation, calling its latest nuclear test a threat to Japan's security.
Abe, speaking to reporters, said Japan absolutely could not tolerate North Korea's nuclear testing.
China plans to summon North Korea's ambassador in Beijing to the Foreign Ministry to lodge a strong protest, spokeswoman Hua Chunying told reporters at a daily briefing Wednesday. China made a similar protest after the North's last nuclear test in 2013.
"North Korea should stop taking any actions which would worsen the situation on the Korean peninsula," Hua said.
French President Francois Hollande and Australian Foreign Minister Julie Bishop were among the other world leaders who condemned North Korea's announcement of a nuclear test.
North Korea has made claims about its nuclear and missile programs in the past that simply have not held up to investigation.
James Martin Center for Nonproliferation Studies
While a fourth nuclear test had been long expected, the latest test came as a surprise, both in its purported type and its timing.
In the first indication of a possible test, the U.S. Geological Survey measured an earthquake Wednesday morning with a magnitude of 5.1. An official from the Korea Metrological Administration, South Korea's weather agency, said the agency believed the earthquake was caused artificially, based on an analysis of the seismic waves and because it originated 30 miles north of Kilju, the northeastern area where North Korea's main nuclear test site is located. The country conducted all three previous atomic detonations there.
North Korea hadn't conducted an atomic explosion since early 2013, and Kim did not mention the country's nuclear weapons in his New Year's speech. Some outside analysts speculated that Kim was worried about deteriorating ties with China, the North's last major ally, which has shown signs of greater frustration at provocations and a possible willingness to allow strong U.N. sanctions.
The size of Wednesday's quake is bigger than seismic activity reported in previous tests. South Korea's Yonhap news agency reported that quake monitoring agencies detected magnitudes of seismic activity of 3.7 in 2006; 4.5 in 2009 and 4.9 in 2013.
After the North's third atomic test, in February 2013, Pyongyang launched a campaign that included threats to launch a nuclear attack on the United States and Seoul. North Korea claimed in 2013 that it had scrapped the 1953 armistice that ended fighting in the Korean War. In September, Pyongyang announced it had revamped and restarted all its atomic bomb fuel production plants.
Pyongyang says its nuclear weapons program is necessary to defend itself against the United States. North Korea under leader Kim Jong Un has pledged to bolster its nuclear arsenal unless Washington scraps what Pyongyang calls a hostile policy.
Washington sees North Korea's pursuit of nuclear weapons and ballistic missiles as a threat to world security and to its Asian allies, Japan and South Korea.
North Korea, analysts say, needs nuclear tests for practical military reasons. Such tests advance its aims to build nuclear-tipped missiles that can be used as deterrents against its enemies — and especially against the United States, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
Researchers at the U.S.-Korea Institute at Johns Hopkins University said in December that satellite images showed North Korea was excavating a new tunnel at Punggye-ri, according to The Japan Times.
“While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea’s ability to conduct additional detonations over the coming years if it chooses to do so,” they said at the time.
The Korean Peninsula remains in a technical state of war since the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 U.S. troops are stationed in South Korea as deterrence against potential aggression from North Korea.
Al Jazeera and wire services