SEOUL: North Korea conducted its sixth and most powerful nuclear test on Sunday, which it said was an advanced hydrogen bomb for a long-range missile, marking a dramatic escalation of the regime's stand-off with the United States and its allies.
The test drew swift international condemnation, including from U.S. President Donald Trump, who described North Korea as a "rogue nation" and said its actions "continue to be very hostile and dangerous to the United States".
"Appeasement with North Korea will not work, they only understand one thing," he said in messages on Twitter.
The White House said Trump's national security team was "monitoring this closely" and the president will convene a meeting of his advisers later on Sunday.
Chinese President Xi Jinping and Russian President Vladimir Putin, who met on the sidelines of a BRICS summit in China, agree to "appropriately deal" with the North Korean nuclear test, the Xinhua news agency reported.
Hours before the test, Trump had talked by phone with Japanese Prime Minister Shinzo Abe about the "escalating" nuclear crisis in the region.
The U.S. president has previously vowed to stop North Korea developing nuclear weapons and said he would unleash "fire and fury" on the regime if it threatened U.S. territory.
North Korea, which carries out its nuclear and missile programmes in defiance of United Nations Security Council resolutions and sanctions, said on state television that the hydrogen bomb test ordered by leader Kim Jong Un had been a "perfect success".
The bomb was designed to be mounted on its newly developed intercontinental ballistic missile (ICBM), the North said.
The test had registered with international seismic agencies as a manmade earthquake near a test site in the North. Japanese and South Korean officials said it was around 10 times more powerful than the tremor picked up after North Korea's last nuclear test a year ago.
There was no independent confirmation that the detonation was a hydrogen bomb, rather than a less powerful atomic device, but Japan's Chief Cabinet Secretary Yoshihide Suga said Tokyo could not rule out such a possibility.
Experts who studied the impact of the earthquake caused by the explosion - measured by the U.S. Geological Survey at magnitude 6.3 - said that there was enough strong evidence to suggest the reclusive state has either developed a hydrogen bomb or was getting very close.
The head of the UN's nuclear watchdog, IAEA Director General Yukiya Amano said the nuclear test was "an extremely regrettable act" that was "in complete disregard of the repeated demands of the international community".
South Korean President Moon Jae-in said Seoul would push for strong steps to further isolate the North, including new UN sanctions. Japan also raised the prospect of further sanctions, saying curbs on North Korea's oil trade would be on the table.
China, North Korea's sole major ally, said it strongly condemned the nuclear test and urged Pyongyang to stop its "wrong" actions.
The United States has repeatedly urged Beijing to do more to rein in its neighbour, but Beijing has lambasted the West and its allies in recent weeks for suggesting that it is solely responsible for doing so. It has said military drills by South Korea and the United States on the Korean peninsula have done nothing to lessen tensions.
Under third-generation leader Kim, North Korea has been pursuing a nuclear device small and light enough to fit on a long-range ballistic missile, without affecting its range and making it capable of surviving re-entry.
North Korea claimed in January last year to have tested a miniaturised hydrogen bomb, also known as a thermonuclear device, but outside experts were sceptical, suggesting it could have been a "boosted device", an atomic bomb that uses some hydrogen isotopes to increase its explosive yield.
A U.S. official who studies North Korea's military and politics said it was too early to determine if the test supported the North's claim that it had succeeded in developing a thermonuclear weapon, "much less one that could be mounted on an ICBM and re-enter Earth's atmosphere without burning up".
The test comes amid heightened regional tension following Pyongyang's two tests of ICBMs in July that potentially could fly about 10,000 km (6,200 miles), putting many parts of the U.S. mainland within range.
One expert said the size of Sunday's detonation meant it was possible it could be a hydrogen bomb test.
"The power is 10 or 20 times or even more than previous ones," said Kune Y. Suh, a nuclear engineering professor at Seoul National University. "That scale is to the level where anyone can say (it was) a hydrogen bomb test."
AIR RAID SIRENS
When the test took place, people in the Chinese city of Yanji, on the border with North Korea, said they felt a tremor that lasted roughly 10 seconds, followed by an aftershock.
"I was eating brunch just over the border here in Yanji when we felt the whole building shake," said Michael Spavor, director of the Paektu Cultural Exchange, which promotes business and cultural ties with North Korea. "It lasted for about five seconds. The city air raid sirens started going off."
Earthquakes triggered by North Korean nuclear tests have gradually increased in magnitude since Pyongyang's first test in 2006, indicating the isolated country is steadily increasing the destructive power of its nuclear technology.
The Comprehensive Test Ban Treaty Organisation (CTBTO) in Vienna said it had detected an "unusual seismic event" in North Korea that was larger than previous nuclear tests.
"North Korea's mission is quite clear when it comes to this latest atomic test: to develop a nuclear arsenal that can strike all of Asia and the U.S. homeland," Harry Kazianis, director of defence studies at the conservative Center for the National Interest in Washington, said.
"This test is just another step towards such a goal. None of us should be shocked by Pyongyang's latest actions."
Hours before the test, North Korea's state news agency KCNA had released pictures showing Kim Jong Un inspecting a silver-coloured, hourglass-shaped warhead during a visit to the country's nuclear weapons institute, accompanied by scientists.
The shape shows a marked difference from pictures of the ball-shaped device North Korea released in March last year, and appears to indicate the appearance of a two-stage thermonuclear weapon, said Lee Choon-geun, senior research fellow at state-run Science and Technology Policy Institute.
KCNA said North Korea "recently succeeded" in making a more advanced hydrogen bomb.
"All components of the H-bomb were homemade and all the processes ... were put on the Juche basis, thus enabling the country to produce powerful nuclear weapons as many as it wants," KCNA quoted Kim as saying.
Juche is North Korea's homegrown ideology of self-reliance that is a mix of Marxism and extreme nationalism preached by state founder Kim Il Sung, the current leader's grandfather. It says its weapons programmes are needed to counter U.S. aggression.
Impoverished North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.
(Additional reporting by Elaine Lies, Kiyoshi Takenaka, Tim Kelly, Takaya Yamaguchi and Nobuhiro Kubo in Tokyo, Jane Chung, Yuna Park, Ju-min Park and James Pearson in Seoul, Sue-Lin Wong in Yanji and Steve Holland and David Brunnstrom in Washington and Shadia Nasralla in Vienna; Writing by Alex Richardson and Raju Gopalakrishnan; Editing by Nick Macfie and Neil Fullick)