The secretive communist state said its sixth nuclear test was a "perfect success", hours after seismologists had detected an earth tremor.
Pyongyang 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.
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.
