North Korea says it tested a hydrogen bomb

North Korea said Wednesday it had conducted a hydrogen bomb test, a defiant and surprising move that, if confirmed, would put Pyongyang a big step closer toward improving its still-limited nuclear arsenal.

A North Korean television anchor said a test of a "miniaturized" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level." State media later crowed that its "H-bomb of justice" lets it stand firm against U.S. aggression.

An estimated explosive yield of 6.0 kilotons and a quake with a magnitude of 4.8 (the U.S. reported 5.1) were detected, lawmaker from South Korea Lee Cheol Woo said the National Intelligence Service told him. That's smaller than the estimated explosive yield of 7.9 kilotons and a quake with a magnitude of 4.9 that were reported after the 2013 nuclear test, he said, and only a fraction of the hundreds of kilotons that a successful H-bomb test's explosion would usually yield. Even a failed H-bomb detonation typically yields tens of kilotons, the NIS told Lee, who sits on the parliament's intelligence committee.

A miniaturized H-bomb can trigger a weak quake magnitude, but only the U.S. and Russia have such H-bombs, Lee cited the NIS as saying.

The U.S. Air Force has aircraft designed to detect the evidence of a nuclear test, and such aircraft could be deployed from a U.S. base on the Japanese island of Okinawa to search for clues. Japanese media said Tokyo has also mobilized its own reconnaissance aircraft for sorties over the Sea of Japan to try to collect atmospheric data.