North Korea conducted its sixth and most powerful nuclear test on Sunday, which it said was a successful detonation of an advanced hydrogen bomb, in a dramatic escalation of the isolated state’s stand-off with the United States and its allies.

The announcement from Pyongyang came a few hours after international seismic agencies detected a manmade earthquake near the North’s test site, which Japanese and South Korean officials said was around 10 times more powerful than the tremor picked up after its last test a year ago.

There was no independent confirmation that the detonation was a hydrogen bomb rather than an atomic device.

The test is a direct challenge to U.S. President Donald Trump, who hours earlier had talked by phone with Japanese Prime Minister Shinzo Abe about the “escalating” nuclear crisis in the region and has previously vowed to stop North Korea developing nuclear weapons that could threaten the United States.

North Korea, which carries out its nuclear and missile programmes in defiance of United Nations Security Council resolutions and sanctions, said in an announcement on state television that a hydrogen bomb test ordered by leader Kim Jong Un was a “perfect success” and a “meaningful” step in completing the country’s nuclear weapons programmes.

The bomb was designed to be mounted on its newly developed intercontinental ballistic missile (ICBM), the North said.

China, North Korea’s sole major ally, said it strongly condemned the nuclear test. The United States has repeatedly urged Beijing to do more to rein in its neighbour.

South Korean President Moon Jae-in said Seoul would push for strong steps to further isolate the North, including new UN sanctions, news agency Yonhap reported.

Japan also raised the prospect of further sanctions, with Chief Cabinet Secretary Yoshihide Suga saying that curbs on North Korea’s oil trade would be on the table.

A U.S. official who studies North Korea’s military and politics said it was too early to determine if a test supported the North’s claim that it has 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 latest nuclear test comes amid heightened regional tension following Pyongyang’s two tests of intercontinental ballistic missiles (ICBM) in July that potentially could fly about 10,000 km (6,200 miles), putting many parts of the mainland United States within range.

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