Observers in recent years believed that North Korea may have been working toward, but didn't yet have the capability to produce, a thermonuclear bomb, which can be hundreds of times more powerful than an atomic bomb.
North Korean leader Kim Jong Un made the claim, according to state media outlet KCNA, while touring a historic weapons industry site in the country, CNN reported.
The communist country has been turned "into a powerful nuclear weapons state ready to detonate self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation," Kim said, according to the KCNA report.
An expert who spoke to South Korea's Yonhap news agency responded skeptically.
"It's hard to regard North Korea as possessing an H-bomb. I think it seems to be developing it," said Lee Chun-geun, a research fellow at the Science and Technology Policy Institute, according to Yonhap in a report Thursday.
Chang Yong-seok, a researcher at the Seoul National University Institute for Peace and Unification Studies, told Yonhap that this appears to be the first time that Kim claimed to have a hydrogen bomb.
Unlike atomic bombs, which employ fission, thermonuclear bombs release power from uncontrolled fusion reactions.