The state news agency released pictures of leader Kim Jong-un inspecting what it said was a new hydrogen bomb.
There has been no independent verification of the claims.
International experts say the North has made advances in its nuclear weapons capabilities but it is unclear if it has successfully miniaturised a nuclear weapon it can load on to a missile.
State news agency KCNA said Kim Jong-un had visited scientists at the nuclear weapons institute and "guided the work for nuclear weaponisation".
