SEOUL - North Korean leader Kim Jong-Un has suggested his nuclear-armed state has developed a hydrogen bomb, a move that would signal a major step forward in its nuclear weapons capabilities. During a recent inspection tour of a historical military site, Kim mentioned that North Korea was already a “powerful nuclear weapons state ready to detonate self-reliant A-bomb and H-bomb to reliably defend its sovereignty”, the North’s official KCNA news agency said Thursday.

The White House on Thursday poured cold water on leader Kim Jong-Un’s suggestion that North Korea has developed a hydrogen bomb.

Spokesman Josh Earnest said the White House had concerns about the “destabilizing actions” of the regime, but said available information “calls into serious question” claims that Pyongyang has a thermonuclear device. During a recent inspection tour of a historical military site, Kim mentioned that North Korea was already a “powerful nuclear weapons state ready to detonate self-reliant A-bomb and H-bomb to reliably defend its sovereignty,” the North’s official KCNA news agency said Thursday.

North Korea has already tested three atom bombs, which rely on nuclear fission. A hydrogen, or thermonuclear, device uses fusion in a chain reaction that results in a far more powerful explosion. Moreover, North Korean leader Kim Jong Un appeared on Thursday to claim his country has developed a hydrogen bomb, a step up from the less powerful atomic bomb, but the United States and outside experts were sceptical.

Kim made the comments as he toured the Phyongchon Revolutionary Site, which marks the feats of his father who died in 2011 and his grandfather, state founder and eternal president, Kim Il Sung, the official KCNA news agency said.

The work of Kim Il Sung “turned the DPRK into a powerful nuclear weapons state ready to detonate a self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation,” KCNA quoted Kim Jong Un as saying.

DPRK are the initials of the isolated North’s official name, the Democratic People’s Republic of Korea. A hydrogen bomb, also known as a thermonuclear bomb, uses more advanced technology to produce a significantly more powerful blast than an atomic bomb. North Korea conducted underground tests to set off nuclear devices in 2006, 2009 and 2013, for which it has been subject to U.N. Security Council sanctions banning trade and financing activities that aid its weapons programme.

In Washington, the White House said it was doubtful that North Korea had developed a hydrogen bomb, but said Pyongyang remained a threat. “At this point, the information that we have access to calls into serious question those claims, but we take very seriously the risk and the threat that is posed by the North Korean regime in their ambitions to develop a nuclear weapon,” White House spokesman Josh Earnest told a regular briefing.

The U.S. State Department repeated a call on North Korea to comply with its international obligations and abandon all nuclear weapons. An official at South Korea’s intelligence agency told Yonhap news agency there was no evidence that the North had hydrogen bomb capacity, and believed Kim was speaking rhetorically.

The Foreign Ministry in China, North Korea’s most important economic and diplomatic backer, said China was dedicated to ensuring the denuclearisation of the Korean peninsula and resolving problems through talks.

“We hope that all sides can do more to ameliorate the situation and make constructive efforts to maintain peace and stability on the peninsula,” ministry spokeswoman Hua Chunying told a daily news briefing when asked about Kim’s remarks. Impoverished North Korea and rich, democratic South Korea remain technically at war after their 1950-53 conflict ended in a truce, not a treaty. The North has threatened to destroy the South and its major ally, the United States, in a sea of flames.

Despite the underground tests, outside experts suspect the North is short of achieving the capability to put a nuclear warhead on a missile, although it has boasted it had succeeded in the miniaturisation of a weapon.

If the hydrogen bomb claim is true, it would indicate advances in the North’s pursuit of nuclear weapons. “I think it’s unlikely that they have an H-bomb at the moment, but I don’t expect them to keep testing basic devices indefinitely, either,” said Jeffrey Lewis of the California-based Middlebury Institute of International Studies at Monterey.

It was possible the North was referring to the technology of boosting the yield of a nuclear device, possibly using fusion fuel, Lewis said. North Korea claimed in 2010 that it had successfully developed fusion technology. Assessing progress of the North’s nuclear programme is difficult because no one outside a close circle of leaders and experts knows what advances have been made.