North Korean leader Kim Jong Un visits the Phyongchon revolutionary site. (KCNA/Reuters)

North Korea has hinted that it has built a hydrogen bomb to “defend its sovereignty and the dignity of the nation,” a development that, if true, would mark an alarming step in its nuclear capabilities.

It is the first time that the regime, which has already conducted three atomic tests, has claimed to have built an exponentially more powerful hydrogen bomb. But analysts were doubtful of Kim Jong Un’s latest bellicose claim, saying the young leader appeared primarily concerned with trying to bolster his legitimacy.

“Do I think they have the capacity to make a hydrogen bomb? I think that’s virtually impossible,” said Daniel Pinkston, an expert on North Korea’s nuclear weapons who is currently at Babes-Bolyai University in Romania.

Kim, the third-generation leader of North Korea, made the claim while visiting the site of a former munitions factory in central Pyongyang. North Korea has become “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,” he said, according to the official Korean Central News Agency.

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.

The installation that Kim was touring, known as the Phyongchon revolutionary site, was visited several times by founding president Kim Il Sung, the current leader’s grandfather, and by Kim Jong Il, his father. Kim Il Sung reportedly test-fired a submachine gun at the shooting range at the site soon after the division of the Korean Peninsula in 1945.

The site routinely appears in official documentaries about revolutionary history and on North Korea’s military industrial complex, according to Michael Madden, an expert on North Korea’s leadership. Photos showed Kim Jong Un inspecting rifles inside a building and speaking outside the building, his aides with notebooks at the ready taking down his every word.

The fact that the statement came not from the National Defense Commission or the politburo, which usually make major pronouncements, but from Kim while he was extolling the achievements of his grandfather and father suggested that he was trying to burnish their legacy and his legitimacy, said Pinkston, a fluent Korean speaker.

Parsing the Korean version, which was slightly different from the English, he said Kim claimed that the sound of his grandfather’s gun was heard at the site 70 years ago, while today North Korea has become a nuclear state “that can make the boom of a hydrogen or atomic bomb.” It did not necessarily mean that North Korea had developed a bomb, he said.

“I'm super-skeptical that they’ve been able to make this scientific advancement,” Pinkston said.

South Korean intelligence specialists also were skeptical and dismissed Kim’s words as rhetoric. “We don’t have any information that North Korea has developed an H-bomb,” Yonhap News Agency quoted an unidentified intelligence official as saying. “We do not believe that North Korea, which has not succeeded in miniaturizing nuclear bombs, has the technology to produce an H-bomb.”

North Korea has a history of making ostentatious claims that cannot be substantiated. In recent months, Pyongyang said it could launch a submarine ballistic missile, had made nuclear warheads small enough to fit on a missile and had restarted its key nuclear facilities at Yongbyon. None of these assertions have been proven. In fact, North Korea appears to have disproved the first claim with a failed missile launch from a submarine last month.

Satellite images suggest that North Korea might be preparing to conduct a nuclear test again, or at least be ready to test again.

North Korea has conducted three nuclear tests since 2006, but despite plenty of saber-rattling, it has not detonated a device since the beginning of 2013. Under Kim Jong Un, however, North Korea has repeatedly asserted itself to be a nuclear state and has refused to return to multilateral talks aimed at persuading it to disarm.

North Korea appears to be building a new tunnel at its nuclear test site, making it more likely that it will test again in the next year, said Jeffrey Lewis, a nonproliferation expert at the Middlebury Institute of International Studies at Monterey, although there are no signs that a test is imminent.

The International Atomic Energy Agency suggested in September that North Korea appeared to be strengthening its nuclear program, although the agency has not been allowed access to the nuclear facilities.

Using satellite imagery, the IAEA observed renovation and construction activity at the main Yongbyon plant, which appears to be consistent with the country’s statements that it is further developing its nuclear capabilities, said Yukiya Amano, director general of the IAEA.

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