TOKYO - North Korea is ready to detonate a hydrogen bomb to "defend its sovereignty and the dignity of the nation," leader Kim Jong Un said Thursday - a threat that remains unsubstantiated.

It is the first time the regime, which has conducted three atomic tests, has claimed to have built a hydrogen bomb.

South Korean intelligence specialists 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 unnamed 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."

But Kim asserted this week that North Korea had 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," while visiting the site of a former munitions factory in central Pyongyang, according to the official Korean Central News Agency.

The site, known in North Korea as the Phyongchon Revolutionary Site, was visited several times by founding president Kim Il Sung and by his son Kim Jong Il, the current leader's grandfather and father. Kim Il Sung reportedly test-fired a sub-machine 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 from Korean news agency showed Kim Jong Un inspecting rifles inside a building, and speaking outside the building, his aides with notebooks at the ready to take down his every word.

"If we struggle in the same spirit with which the workers produced sub-machine guns by their own efforts just after the liberation of the country, when everything was in need, we can further build up our country into a powerful one no enemy dares provoke," the Korean news agency recounted Kim Jong Un as saying.

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 has been proven.