North Korea said it had developed a more advanced nuclear weapon that has "great destructive power" and leader Kim Jong Un had inspected a hydrogen bomb that will be loaded on a new intercontinental ballistic missile (ICBM).

The report by North Korea's official KCNA news agency comes amid heightened regional tension following Pyongyang's test launch of two ICBM-class missiles in July that potentially had a range of about 10,000 km (6,200 miles) that could hit many parts of the mainland United States.

Under its current leader, North Korea has pursued work on building nuclear weapons and long-range ballistic missiles that can deliver them at an unprecedented pace, defying UN sanctions and international pressure.

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.

The hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes, with its home-produced components allowing the country to build as many nuclear weapons as it wants, KCNA news agency said.

Mr Kim visited the country's Nuclear Weapons Institute and "watched an H-bomb to be loaded into new ICBM", KCNA said.

"All components of the H-bomb were homemade and all the processes ... were put on the Juche basis, thus enabling the country to produce powerful nuclear weapons as many as it wants, he said."

​Juche is the North Korean government's go-it-alone ideology and is a mix of Marxism and extreme nationalism, as preached by state founder Kim Il Sung, the current leader's grandfather.

Kim Jong Un "set forth tasks to be fulfilled in the research into nukes", KCNA said, but it made no mention of plans for a sixth nuclear test.

North Korea last year conducted its fourth and fifth nuclear tests, saying the fourth in January 2016 was a successful hydrogen bomb test, although outside experts questioned whether it was a fully-fledged hydrogen bomb.

The fifth nuclear test in September 2016 was measured to be possibly North Korea's biggest detonation ever, but the earthquake it caused was still not believed to be big enough to demonstrate a thermonuclear test.

US officials have told Reuters that while North Korea has had parts in place for a nuclear detonation going back several months, no new activity has been seen recently at its known nuclear test site in Punggye-ri in its northeastern region.