A large earth tremor detected in North Korea was the country's sixth nuclear test, Japan's foreign minister says, reports BBC.
"The government confirms that North Korea conducted a nuclear test after examining information from the weather agency and other information," Taro Kono told reporters.
US seismologists had said the 6.3 magnitude quake in the north-east of the country was a "possible explosion".
It was in the area where the North has conducted previous nuclear tests.
The tremor came 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 Korea officials said the quake took place in Kilju County, where the North's Punggye-ri nuclear test site is situated.
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.
Bruce Bennett, a defence analyst at public policy group the Rand Corporation told the BBC that the size of the tremor was significant.
"If it really does prove out to be 6.3, that's a very big weapon, much bigger," he said.
"It's still not a true hydrogen bomb but it's certainly much closer to that than anything they have ever done before."
He said China would also be concerned about the size of the suspected blast.
"Those Chinese people across the border from that area, they were undoubtedly shaken terribly," he said.