NKorea says it has advanced H-bomb

North Korea says it has a hydrogen bomb with an explosive power that is adjustable up to hundreds of kilotons and can be detonated at high altitudes.

Kim Jong Un was present at a factory when an advanced hydrogen bomb was was loaded onto a missile. (AAP)

North Korean scientists have built an advanced hydrogen bomb that can be mounted onto an inter-continental ballistic missile, state media says.

Pyongyang's official mouthpiece, the Korean Central News Agency (KCNA), said on Sunday leader Kim Jong Un was present at a factory as the nuclear weapon was loaded onto a missile.

The bomb has an explosive power that is adjustable up to "hundreds" of kilotons and can be detonated at high altitudes with its indigenously produced components allowing the country to build as many nuclear weapons as it wants, KCNA reported.

In January 2016, North Korea claimed it had successfully tested a hydrogen bomb, its fourth test of an atomic device, but Western experts doubted it had the power of a hydrogen nuclear explosion.

North Korea escalated regional tensions once again last week by firing a ballistic missile over Japan in defiance of UN resolutions and fresh sanctions imposed last month which are expected to slash its export revenues by a third.

That launch came after Pyongyang tested two intercontinental missiles in July and claimed to have a range of about 10,000km that could hit many parts of the mainland US.

Experts and officials have said North Korea could conduct its sixth nuclear test at any time, and that the reclusive country has maintained a readiness at its nuclear test site to conduct another detonation test at any time.