The UN Security Council says it will begin work immediately on new measures against North Korea, after Pyongyang said it had tested a hydrogen bomb.
The council condemned the test, saying "a clear threat to international peace and security continues to exist".
This is the North's fourth nuclear test since 2006, but if confirmed would be the first of an H-bomb.
However, the US has joined nuclear experts in questioning whether the blast was large enough for such a test.
US White House spokesman Josh Earnest said "initial analysis was not consistent with North Korea's claims of a successful hydrogen bomb test".
He added: "Nothing that has occurred in the last 24 hours has caused the United States government to change our assessment of North Korea's technical and military capabilities.", foreign media reports.