Kenneth W. Ford, an American physicist who worked on the nation’s first hydrogen bomb and last year published an H-bomb memoir, called the North Korean claim highly suspect. “How could a thermonuclear blast trigger such a weak seismic signal?” he said. “I agree with the suspicion that it was not a true H-bomb.”

The club of nations that possess thermonuclear weapons has only five known members: the United States, Russia, Britain, France and China. All are permanent members of the United Nations Security Council.

David Albright, president of the Institute for Science and International Security, a Washington research group, said North Korea “may be bluffing” in making very large claims for what was actually a small atom bomb. “This possibility,” he said, “should be carefully considered.”

South Korean experts put the blast’s energy as equivalent to six kilotons of high explosives. In contrast, the atomic bomb dropped on Hiroshima was nearly three times as powerful, with a force of about 15 kilotons.