North Korea conducts 'perfect' H-bomb test

North Korea has conducted its sixth and most powerful nuclear test, which it says is an advanced hydrogen bomb for a long-range missile, marking a dramatic escalation of the stand-off with the United States and its allies.

Sunday's 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 nuclear test a year ago.

There was no independent confirmation that the detonation, which drew swift international condemnation, was a hydrogen bomb rather than a less powerful atomic device.

North Korea, which carries out its nuclear and missile programs 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".

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

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".

French President Emmanuel Macron said the international community must react firmly to this "new provocation", and 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.

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 US 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 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 latest nuclear test comes amid heightened regional tension following Pyongyang's two tests of ICBMs in July that potentially could fly about 10,000km, putting many parts of the US mainland within range.

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.

One expert said the size of Sunday's detonation, measured by the US Geological Survey at magnitude 6.3, 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 a hydrogen bomb test."

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.

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.

Kim "watched an H-bomb to be loaded into new ICBM" and "set forth tasks to be fulfilled in the research into nukes", KCNA said.

Tensions on the Korean peninsula have been high since last month when North Korea threatened to launch missiles into the sea near the strategically located US Pacific territory of Guam after Trump said Pyongyang would face "fire and fury" if it threatened the United States.

US President Donald Trump and Japanese Prime Minister Shinzo Abe spoke by phone and said that in face of an "escalating" situation with North Korea that close co-operation between their countries and with South Korea was needed, Abe told reporters.

Abe blasted the test, saying North Korea's "nuclear and missile development programs pose a new level of a grave and immediate threat" and "seriously undermines the peace and security of the region."

South Korea said North Korea's defiant test should be met with the "strongest possible" response, including new UN Security Council sanctions to "completely isolate" the country.

Russia's foreign ministry said the test was a defiance of international law and deserved condemnation. It urged all sides involved to hold talks, which it said was the only way to resolve the Korean peninsula's problems.