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 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 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.

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.

North Korea and 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.