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.
North Korea announces hydrogen bomb test
06 January 2016
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North Korea says it has successfully tested a hydrogen bomb amid reports of a tremor near the main nuclear test site.
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