International scepticism and condemnation have greeted North Korea's claim to have successfully carried out an underground hydrogen bomb test.<br />If confirmed, it would be North Korea's fourth nuclear test since 2006 and mark a major upgrade in its capabilities.<br />But nuclear experts have questioned whether the size of the blast was large enough to have been from an H-bomb.<br />UN chief Ban Ki-moon condemned the test "unequivocally", calling it "profoundly destabilising for regional security".<br />South Korea called the test a "grave provocation" but said it was difficult to believe it was an H-bomb.<br />Hydrogen bombs are more powerful and technologically advanced than atomic weapons, using fusion - the merging of atoms - to unleash massive amounts of energy.
