That would make it significantly more worrisome than previous tests, as The Washington Post explains:
Hydrogen, or thermonuclear, bombs are exponentially more powerful and destructive than atomic devices. An atomic bomb uses fission to break up the atomic nucleus and release energy, while a hydrogen or thermonuclear bomb uses fusion to add to the nucleus. This leads to an enormous explosion resulting from an uncontrolled, self-sustaining chain reaction.
But the international community cannot take North Korea at its word, and - as CNN notes - it could take "several days" to figure out whether or not the rogue nation actually detonated a hydrogen bomb. Experts are already skeptical, suggesting that the public has reason to stay calm.
Jeffrey Lewis, a nuclear expert, told the Washington Post that the "explosion looked very similar to past tests and was not enormous, suggesting it was not a hydrogen bomb." Lewis also noted in a Johns Hopkins publication in December that given what's known about North Korea's capacity, building a hydrogen bomb "would seem to be a bit of a stretch."
"Thermonuclear weapons are tricky; making one work requires a bit of test experience," he continued. "A more technically plausible scenario is that North Korea might be experimenting with fusion fuels" to boost the power of an atomic explosion: a scary scenario, certainly, but no H-bomb.
The yield of Wednesday's blast was estimated to be six kilotons, far less than the power that would be expected from a hydrogen bomb and a fraction of the power of the atomic bombs dropped on Hiroshima and Nagasaki.
A statement on North Korean TV, quoted by The Guardian, said the test was of what the government called a "miniaturised hydrogen bomb," but it's unclear exactly what is meant by that. It's possible that could explain some of the discrepancy.
"The nuclear weapons test announced by North Korea undermines regional and international security," NATO's Secretary General said in a statement. But it's not unprecedented, and its power seems to match that of its earlier tests. So while there is reason to be concerned about what may be North Korea's growing nuclear capability, this particular test may not be alarming and novel as it initially seemed.