North Korea on Wednesday claimed it had carried out a nuclear fusion reaction that could lead to a limitless supply of clean energy -- a process that the world's scientists have so far yet to achieve.

Physicists worldwide are striving to develop a nuclear fusion power plant, a project which the International Atomic Energy Agency terms "a great challenge".

But North Korea said it had triumphed using its own technology.

"The successful nuclear fusion marks a great event that demonstrated the rapidly developing cutting-edge science and technology of the DPRK (North Korea)," said Rodong Sinmun, the newspaper of the ruling communist party.

It said the North's experts had worked hard to develop the "safe and environment-friendly new energy" technology their own way.

"Korean style" thermonuclear reaction devices were designed and manufactured as part of the process, it added.

South Korean experts doubted that the North -- which suffers persistent power shortages in everyday life -- has made major progress in a process which potentially promises clean and limitless energy.

Yang Hyung-Lyeol, of South Korea's state-funded National Fusion Research Institute, said: "I don't think the North has any technology that we are not aware of. If so, it would mean the North would be on top of the world."

Nuclear fusion reactions can also be employed to make hydrogen bombs. But Yang said Wednesday's announcement did not seem linked to the North's atomic weapons programme.

Yang said there is little possibility of the technology being used for weapons. "Judging from technological terminology used in the announcement, the development has nothing to do with weaponising."

Yang said he was sceptical the North has technology or facilities on a par with advanced countries.

"North Korea may have began operating a small-scale magnetic nuclear fusion device but you cannot draw any parallel with our own fusion reactor KSTAR and other reactors in the world," he told AFP.

South Korea is a partner in the International Thermonuclear Experimental Reactor (ITER) project to build a fusion power plant by the mid 2030s. Other key members include the United States, European Union members, Japan, Russia, China and India.

"Nuclear fusion research has been jointly conducted by key industrial states in the form of ITER and related technology is quite open in public and shared by many countries," Yang said.

The North said the achievement was made to mark the Day of the Sun -- the anniversary on April 15 of the birth of founding president Kim Il-Sung.

Nuclear fusion expert Lee Choon-Geun of South Korea's state-financed Science and Technology Policy Institute said the North had been conducting research in this field since the early 2000s.

"But it all comes down to budget and facilities. I don't think the North has such facilities as we and other advanced countries have."

The North has for decades had a nuclear weapons programme based on plutonium produced from spent fuel at its Yongbyon reactor. US Secretary of State Hillary Clinton has estimated it has up to six atomic weapons.

Last September it announced for the first time that it had reached the final stage of enriching uranium, a second way of making nuclear bombs.

Six-nation talks aimed at shutting down the North's nuclear programmes have been stalled since December 2008. In April last year the North announced it was quitting the forum.

It staged its second atomic weapons test the following month, incurring tougher United Nations sanctions.