North Korea claims that it has successfully tested a nuclear weapon that could be loaded onto a long-range missile.
Pyongyang said its sixth nuclear test was a “perfect success”, hours after seismologists had detected an earth tremor.
North Korea said it had tested a hydrogen bomb – a device many times more powerful than an atomic bomb.
Analysts say the claims should be treated with caution, but its nuclear capability is clearly advancing.
Pyonyang last carried out a 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.
According to South Korean officials, the latest test took place in Kilju County, where North Korea’s Punggye-ri nuclear test site is situated.
The “artificial quake” was 9.8 times more powerful than the tremor from North Korea’s fifth test, the state weather agency said.
It came hours after Pyongyang said it had miniaturized a hydrogen bomb for use on a long-range missile, and 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.
Initial reports from the US Geological Survey put the tremor at 5.6-magnitude with a depth of 6 miles but this was later upgraded to 6.3-magnitude at 0 miles. This would make it North Korea’s most powerful nuclear test to date.
Japan condemned the test and South Korean President Moon Jae-in convened emergency security council talks.
A series of recent missile tests has caused growing international unease.
In a report on September 3, North Korea’s state news agency KCNA said Kim Jong-un had visited scientists at the nuclear weapons institute and “guided the work for nuclear weaponization”.
The report said: “The institute recently succeeded in making a more developed nuke.”
“He (Kim Jong-un) watched an H-bomb to be loaded into a new ICBM (intercontinental ballistic missile),” it added.
The report carried pictures of Kim Jong-un inspecting the device. It described the weapon as “a multi-functional thermonuclear nuke with great destructive power which can be detonated even at high altitudes”.
International experts say North Korea has made advances in its nuclear weapons capabilities but it is unclear if it has successfully miniaturized a nuclear weapon it can load on to a missile.
North Korea has previously claimed to have miniaturized a nuclear weapon but experts have cast doubt on this. There is also skepticism about North Korea’s claims to have developed a hydrogen bomb, which is more powerful than an atomic bomb.
Hydrogen bombs use fusion – the merging of atoms – to unleash huge amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms.