North Korea has carried out its sixth nuclear test, Japan’s foreign minister says.
“The government confirms that North Korea conducted a nuclear test after examining information from the weather agency and other information,” Taro Kono told reporters.
North Korean state TV says it will make an “important announcement” shortly.
Seismologists earlier detected a tremor in an area where North Korea has conducted previous nuclear tests.
The tremor was detected hours after North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.
State media said the device could be loaded on to a ballistic missile. Neither claim could be independently verified.
South Korean officials said the quake took place in Kilju County, where the North’s Punggye-ri nuclear test site is situated.
The “artificial quake” was 9.8 times more powerful than the tremor from the North’s fifth test, the state weather agency said.
Soon after, South Korean President Moon Jae-in convened an emergency meeting of his national security council.
China’s Earthquake Administration described the tremor as a “suspected explosion”. It said it had detected a second tremor just after the first, of 4.6 magnitude, which it termed “a collapse”.
Initial reports from the US Geological Survey put the tremor at 5.6 magnitude with a depth of 10km (six miles) but this was later upgraded to 6.3 magnitude at 0km. This would make it the North’s most powerful nuclear test to date.
North Korea carried out its last nuclear test in September 2016. It has defied UN sanctions and international pressure to develop nuclear weapons and to test missiles which could potentially reach the mainland US.