North Korea claims it has successfully tested a nuclear weapon that could be loaded onto a long-range missile.
The secretive communist state said its sixth nuclear test was a "perfect success", hours after seismologists had detected an earth tremor.
The government said it had tested a hydrogen bomb - a device many times more powerful than an atomic bomb.
However, analysts say the claims should be treated with caution.
North Korea 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.
South Korean officials said the latest test 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.
South Korean President Moon Jae-in has now convened emergency security council talks.