However, White House officials today said they doubted North Korea's claim. They believe it was more likely an atomic bomb, similar in size to three previous ones tested by North Korea since 2006.
White House spokesman Josh Earnest said initial data from various monitoring sources were "not consistent with North Korean claims of a successful hydrogen bomb test."
Even so, many nuclear arms control experts say the test is worrisome.
"As for the rumor or claim of thermonuclear warhead, I doubt that seriously. The kind of seismic signature suggests something similar to a previous test," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. "That's not to say they are not working on it."
North Korea does have medium-range ballistic missile that can carry conventional weapons as far as South Korea and Japan, and is working on building intercontinental ballistic missiles (ICBMs) that could travel much farther, according to Reif.
The country has these missiles, but they aren't working yet, he said.