North Korea conducted its sixth nuclear test, the Japanese government said on Sunday, after an natural disaster was detected near the site where the reclusive nation has previously detonated nuclear explosions.
Reports have suggested that Pyongyang could soon carry out a sixth nuclear test, but the respected 38 North website said last week that satellite imagery of the Punggye-ri test site showed no evidence that a blast was imminent.
The head of the International Atomic Energy Agency, the global nuclear watchdog, said in a statement that North Korea's latest nuclear test was "an extremely regrettable act".
The alleged artificial quake was identified at 12.30am local time in the areas around the North Hamgyeong province, according to South Korea's weather agency.
North Korea's state media has claimed the country has developed a "more developed" nuclear weapon it called a hydrogen bomb, a boast that follows intercontinental ballistic missile tests that sparked global condemnation.
North Korea's last nuclear test was in September 2016.More news: Tamil Nadu girl, who filed case against Neet, ends life
Experts estimate today's nuclear explosion produced a yield stronger than 100 kilotons, which, if correct, would dwarf the rogue nation's last nuclear test of around 20 kilotons in September previous year.
It is seeking to develop a nuclear weapon small enough to be mounted on an ICBM.
South Korea's President, Moon Jae-in, has called a meeting of his National Security Council.
Pyongyang's only major ally, China, said it strongly condemned the detonation.
North Korea watchers say that could be imminent.
The test comes just hours after the country released images of Kim inspecting what it said was a hydrogen bomb ready to be fitted atop a missile with intercontinental capabilities.More news: Super Eagles offer support to ill Carl Ikeme
"If North Korea did indeed conduct a nuclear test, we absolutely can not tolerate and must protest firmly".
The United States Geological Survey detected an "explosion" with a magnitude of 6.3.
State media reports that Kim Jong Un was on-hand to oversee the country's more advanced take on the hydrogen bomb.
North Korea further raised regional tensions on Tuesday by launching an intermediate-range ballistic missile over Japan, drawing worldwide condemnation.
South Korea's spy agency recently told lawmakers that the North remains ready to conduct its sixth nuclear test any time.
Experts are sceptical of the claim that Pyongyang has mastered hydrogen technology, although it is nearly impossible to verify statements about its highly secret weapons programme. "The North Koreans have said this was a goal for a long time".More news: Canadian GDP sees strongest half-year growth in 15 years: StatsCan