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 already 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, the current leader’s grandfather, and by his father, Kim Jong Il. 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. He stressed the need to develop the country’s munitions industry, the report said.

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.

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 convincing it to disarm.

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

Although there are no signs that a test is imminent, North Korea appears to be building a new tunnel at its nuclear test site, said Jeffrey Lewis, a nonproliferation expert at the Middlebury Institute of International Studies at Monterey. This makes it “more likely that they will conduct a test in the coming year,” he said.

The International Atomic Energy Agency in September suggested that North Korea did appear to be strengthening its nuclear program, although it had not been allowed access to the nuclear facilities.

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