Does North Korea have a hydrogen bomb?

Seoul: North Korean leader Kim Jong-Un has hinted that his nuclear-armed state has developed a hydrogen bomb, a move that would signal a major step forward in its nuclear weapons capabilities.

During a recent inspection tour of a historical military site, Kim mentioned that North Korea was already a "powerful nuclear weapons state ready to detonate self-reliant A-bomb and H-bomb to reliably defend its sovereignty", the North`s official KCNA news agency said on Thursday.

North Korea has already tested three atom bombs, which rely on nuclear fission.

A hydrogen, or thermonuclear device, uses fusion in a chain reaction that results in a far more powerful explosion.

North Korea has hinted before at the possession of "stronger, more powerful" weapons, but Kim`s remarks were believed to be the first direct reference to an H-bomb.

The North has made many unverifiable claims about its nuclear weapons strength, including the ability to strike the US mainland which most experts dismiss -- at least for now.

In September, the Washington-based Institute for Science and International Security (ISIS) had raised a red flag over what appeared to be a new "hot cell" facility under construction at the North Korea`s main Yongbyon nuclear complex.

Analysts at the think-tank said satellite images suggested it could be an isotope separation facility, capable of producing tritium.

Tritium is a key component in the design of more thermonuclear weapons with far greater yields than those made only of plutonium and uranium.

The North`s first two nuclear tests in 2006 and 2009 were of plutonium devices, while the third was believed -- though not confirmed -- to have used uranium as its fissile material.

"Whether North Korea can make nuclear weapons using tritium is unknown although we believe that it remains a technical problem North Korea still needs to solve," ISIS said at the time.

"Solving this problem would likely require more underground nuclear tests," it added.