North Korea’s leader Kim Jong-un has claimed his country possesses a hydrogen bomb, as well as an atomic bomb, reported Reuters.

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 Democratic People’s Republic of Korea 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 Mr Kim as saying.

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 has carried out three underground nuclear tests before, but experts cast doubt over the latest suggestions.

John Nilsson-Wright, Head of the Asia Programme at Chatham House, was sceptical, saying it fitted into a previous pattern of bold claims from the North Korean leader.

The comments were likely to be an “attention-grabbing effort to assert North Korean autonomy and his own political authority”, he told the BBC.

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.

The Foreign Ministry in China, North Korea’s most important diplomatic backer, said China was dedicated to ensuring the decentralization 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.

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.