The development — a major milestone in the pursuit of limitless clean energy  — was made recently at the government-funded Lawrence Livermore National Laboratory in California, sources told the Washington Post.

The net gain reaction, which is considered to be a "holy grail" of zero-carbon power, is a major step towards being able to create a technology one day that could offer a clean and virtually limitless supply of energy.

Energy Secretary Jennifer Granholm is expected to announce the results of the fusion experiment at a media event on Tuesday.

A lab official said researchers were still finalizing their findings and nothing would be confirmed until the scheduled event.

The energy department also wouldn't comment.

Scientists have been trying since the 1950s to replicate the same nuclear reaction that powers the sun as part of the global quest to offer clean energy and combat climate change.

But, until now, no researchers have been able to produce more energy from the fusion reaction than it consumes.