North Korea says it has successfully tested a hydrogen bomb amid reports of a tremor near the main nuclear test site.
State media announced the test after monitors detected a 5.1 magnitude quake close to the Punggye-ri site.
The North is thought to have conducted three previous underground nuclear tests there since 2006.
A hydrogen bomb uses fusion to create a blast far more powerful than that of a more basic atomic bomb.
This test was apparently a hydrogen bomb, a step up in destructive power from the plutonium used in previous tests. It gives more explosive power for a lighter weight.
In a surprise announcement a newsreader on North Korean state TV said: “The republic’s first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016.”
Last month, North Korean leader Kim Jong-un said Pyongyang had developed a hydrogen bomb, although international experts were sceptical.