Image copyright Reuters/KCNA Image caption State media said Kim Jong-un "watched an H-bomb to be loaded into a new ICBM"

North Korea says it has developed a more advanced nuclear weapon that can be loaded on to a ballistic missile.

The state news agency released pictures of leader Kim Jong-un inspecting what it said was a new hydrogen bomb.

There has been no independent verification of the claims.

International experts say the North has made advances in its nuclear weapons capabilities but it is unclear if it has successfully miniaturised a nuclear weapon it can load on to a missile.

Pyongyang has defied UN sanctions and international pressure to develop nuclear weapons and to test missiles which could potentially reach the mainland US.

State news agency KCNA said Kim Jong-un had visited scientists at the nuclear weapons institute and "guided the work for nuclear weaponisation".

"The institute recently succeeded in making a more developed nuke," the report said, adding: "He (Kim Jong-un) watched an H-bomb to be loaded into a new ICBM (intercontinental ballistic missile)."

The report carried pictures of the leader inspecting the device. It described the weapon as "a multi-functional thermonuclear nuke with great destructive power which can be detonated even at high altitudes".

Defence expert Melissa Hanham, of the Middlebury Institute for International Studies in California, said that the North's claims could not be verified from the photographs alone.

"We don't know if this thing is full of styrofoam, but yes, it is shaped like it has two devices," she said on Twitter. Hydrogen bombs detonate in two stages.

She added: "The bottom line is that they probably are going to do a thermonuclear test in the future, we won't know if it's this object though."

Image copyright KCNA Image caption North Korea's missile launches have caused growing international alarm

North Korea has carried out a series of missile tests in recent months, including weapons that put the mainland US in range.

Last week it fired a missile over Japan in a move Prime Minister Shinzo Abe called an "unprecedented" threat to his country.

Mr Abe and US President Donald Trump spoke by phone after the latest report emerged. The pair agreed more pressure needed to be put on North Korea, Mr Abe said.

The North has previously claimed to have miniaturised a nuclear weapon but experts have cast doubt on this. There is also scepticism about the North's claims to have developed a hydrogen bomb, which is more powerful than an atomic bomb.

Hydrogen bombs use fusion - the merging of atoms - to unleash huge amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms.

North Korea has conducted five nuclear tests. Its most recent, and most powerful, came in September last year.

Correspondents say that although North Korea could conduct its sixth test at any time, there has been no recent activity at its Punggye-ri test site.

North Korea's missile programme: