North Korea detonated its sixth and most powerful nuclear test on Sunday, Japan and South Korea said, hours after Pyongyang said it had developed an advanced hydrogen bomb that possesses "great destructive power".

Japanese and South Korean meteorological officials said an earthquake detected near the North's test site - measured by the U.S. Geological Survey (USGS) at magnitude 6.3 - was around 10 times more powerful than previous detonations.

North Korea said it would make an important announcement at 0630 GMT.

The move 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.

Japan immediately raised the prospect of further sanctions against the isolated North, with Chief Cabinet Secretary Yoshihide Suga saying that curbs on its oil trade would be on the table.

A U.S. official who studies North Korea's military and politics said that seismic data on the tremors was being analysed, although the location suggested another nuclear test.

The official, who spoke on the condition of anonymity, said it was too early to determine if a test, if there was one, supported the North's claim that 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 hydrogen bomb report by North Korea's official KCNA news agency 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.

Under third-generation leader Kim Jong Un, 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 into the Earth's atmosphere.

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 a hydrogen bomb test."

AIR RAID SIRENS

Witnesses 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. China said it had detected a second, 4.6 magnitude quake with near identical coordinates eight minutes later.

"I was eating brunch just over the border here in Yanji when we felt the whole building shake," 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."

South Korea's military said the first earthquake "appeared to be manmade". A meeting of Seoul's National Security Council has been convened, national news agency Yonhap reported.

Japan said it had concluded there was a nuclear test.

"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," he added.

Kazianis also pointed out that Pyongyang's latest actions should not come as a surprise.

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 improving the destructive power of its nuclear technology.

After the fifth nuclear test in September, USGS measured a magnitude of 5.3. while South Korean monitors said the blast caused a 5.0 magnitude earthquake.

North Korea, which carries out its nuclear and missile programmes in defiance of UN Security Council resolutions and sanctions, "recently succeeded" in making a more advanced hydrogen bomb that will be loaded on to an ICBM, KCNA said.

"The H-bomb, the explosive power of which is adjustable from tens kiloton to hundreds kiloton, is a multi-functional thermonuclear nuke with great destructive power," KCNA said.

"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.

North Korea offered no evidence for its latest claim, and Kim Dong-yub, a military expert at Kyungnam University's Institute of Far Eastern Studies in Seoul, was sceptical.

"Referring to tens to hundreds of kilotons, it doesn't appear to be talking about a fully fledged H-bomb. It's more likely a boosted nuclear device," Kim said, referring to an atomic bomb which uses some hydrogen isotopes to boost explosive yield.

A hydrogen bomb can achieve thousands of kilotons of explosive yield - massively more powerful than some 10 to 15 kilotons that North Korea's last nuclear test in September was estimated to have produced, similar to the bomb dropped on Hiroshima, Japan, in 1945.

HOURGLASS-SHAPED DEVICE

Kim Jong Un, who visited the country's nuclear weapons institute, "watched an H-bomb to be loaded into new ICBM" and "set forth tasks to be fulfilled in the research into nukes," KCNA said.

Pictures released by the agency showed Kim inspecting a silver-coloured, hourglass-shaped warhead in the visit accompanied by nuclear 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, or a hydrogen bomb, said Lee Choon-geun, senior research fellow at state-run Science and Technology Policy Institute.

"The pictures show a more complete form of a possible hydrogen bomb, with a primary fission bomb and a secondary fusion stage connected together in an hourglass shape," Lee 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 U.S. Pacific territory of Guam after Trump said Pyongyang would face "fire and fury" if it threatened the United States.

North Korea further raised regional tensions on Tuesday by launching an intermediate-range ballistic missile over Japan, drawing international condemnation.

Trump and Abe spoke by phone and said that in face of an "escalating" situation with North Korea that close cooperation between their countries and with South Korea was needed, Abe told reporters.

The United States has repeatedly urged China, the North's sole major ally, to do more to rein in its neighbour.

It is important to note that 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.