Instead of splitting the atom -- the principle behind the atomic bomb and present-day nuclear reactors -- fusion entails ramming the nuclei of light atomic elements together to make heavier elements and in the process release huge amounts of energy.

In essence, it is the same principle that powers the stars. In the Sun, mighty gravitational forces crush hydrogen atoms together to produce helium, with solar energy the byproduct.

Replicating stellar power on Earth, though, means overcoming a daunting array of technical and financial hurdles and drawing on expertise from many fields and different countries.

So the sketchy claims by the impoverished, enclosed Communist state are bound to be eyed with scepticism.

Rodong Sinmun, the newspaper of Pyongyang's ruling communist party, said the North's experts had developed nuclear fusion using a "Korean-style" thermonuclear device.

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

Under fusion, a huge jolt of heat, to nearly 100 million degrees Celsius (180 million degrees Fahrenheit) would kickstart the process, fusing atomic nuclei and containing them in a charged gas called a plasma.

Getting the process started is only one problem. Another is how to how to sustain it and contain the plasma so that the cloud of particles do not escape.

Then there is the big energy equation -- the cost in energy it takes to pump up the plasma to such high temperatures in comparison with the yield this brings.

So far, despite steady improvements, no one has achieved a self-sustaining fusion event longer than a few minutes and input/yield ratios remain low.

Current projects in nuclear fusion are only at the early experimental stage and have each required commitments of billions of dollars just to be initiated.

A European-led initiative is the International Thermonuclear Experimental Reactor (ITER), located Cadarache, southern France, scheduled to start plasma experiments in 2018 and if successful lead to a commercial reactor in the 2030s.

The backers in the 10-billion-euro scheme are the European Union (EU), which has a 45-percent share, China, India, South Korea, Japan and Russia as well as the United States.

The idea is to have fusion in a reactor fuelled by two isotopes of hydrogen -- deuterium and tritium -- with helium as the waste product in addition to the energy.

The plasma would be contained in a magnetic field in a doughnut-shaped vessel called a tokamak.

Looking at the key issue of fusion ignition, the United States has built the world's largest laser, a 3.5-billion-dollar behemoth covering the size of two football pitches, at the Lawrence Livermore National Laboratory in California.

The idea is for 192 laser beams to zap fuel tiny pellets of beryllium, plastic or high-density carton, so compressing the fuel that temperatures of 111 million C (200 million F) are briefly reached.

Tested for the first time in January, the lasers fire 40 times more power than the average consumption of the entire planet, albeit for only a few nanoseconds.

Fusion's supporters say the abundance of raw materials is almost infinite and the energy itself and its waste products are far safer than with fission, for there is no critical mass. Critics say the research billions would be better spent on improving current clean renewable sources, such as wind, solar and wave energy.

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