Still, it was unclear whether the North had in fact detonated such a weapon, a far more powerful type of nuclear device than the atomic bombs it has tested in the past. And analysts were skeptical that Pyongyang had really developed the capability to mount one on an ICBM.

The United States Geological Survey estimated that the tremor set off by the blast, detected at 12:36 p.m. at the Punggye-ri underground test site in northwestern North Korea, had a magnitude of 6.3.

The South Korean Defense Ministry’s estimate was much lower, at 5.7, but even that would mean a blast “five to six times” as powerful as the North’s last nuclear test, a year ago, said Lee Mi-sun, a senior analyst at the South Korean Meteorological Administration.