SEOUL (Reuters) - North Korean leader Kim Jong Un on Thursday appeared to say his country has developed a hydrogen bomb, a step up from the less powerful atomic bomb, but the United States and outside experts were skeptical.

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 program.

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 denuclearization 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 miniaturization of a weapon.

North Korean leader Kim Jong Un (front) visits the Phyongchon Revolutionary Site, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang December 10, 2015. REUTERS/KCNA

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.

Nuclear expert David Albright, head of Washington’s Institute for Science and International Security, said there had been concerns for several years that North Korea has been pursuing thermo-nuclear weapons.

However, any such device it might be working on would be cruder and more old-fashioned than the classical H-bomb and would require larger nuclear tests than North Korea has yet conducted.

“I am skeptical that they could have succeeded by now,” he said.

Assessing progress of North Korea’s nuclear program is particularly difficult because no one outside a close circle of leaders and experts knows what advances have been made.