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, Reuters reports.

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.”