BEIJING—A successful test of a North Korean hydrogen bomb, if confirmed, would be a milestone in the country’s nuclear program, potentially yielding a more powerful weapon than the kind it has tested in the past.

Questions remain, however, over what exactly North Korea tested Wednesday and its significance. The power of the blast underground near a known test site in the country’s northeast appears to have been roughly the same as that of a device tested in 2013 that was thought to be weaker than a hydrogen bomb.

A hydrogen bomb—technically known as a “thermonuclear weapon”—usually uses a smaller, primary atomic explosion to ignite a secondary, much larger blast. The first stage is based on nuclear fission—the splitting of atoms—and the second on nuclear fusion, which combines atoms, smashing them together and unleashing more energy. Additional stages can be added to increase its destructive force.

That makes the H-bomb far more powerful than early nuclear weapons that typically used a single-stage blast based only on nuclear fission. Those weapons are known as “pure fission” devices and are thought to have been used in all of North Korea’s three previous nuclear tests, which it said involved atomic bombs.

The first atomic weapons built and the only ones used in war—in the U.S. attacks on Japan in 1945—were all of the “pure fission” variety. During the Cold War, the power of such weapons rapidly increased.