The US Department of Energy said Sunday it would announce a "major scientific breakthrough" this week, after media reported a federal laboratory had recently achieved a major milestone in nuclear fusion research.

The Financial Times reported Sunday that scientists in the California-based Lawrence Livermore National Laboratory had achieved a "net energy gain" from an experimental fusion reactor.

That would represent the first time that researchers have successfully produced more energy in a fusion reaction — the same type that powers the Sun — than was consumed during the process, a potentially major step in the pursuit of zero-carbon power.

Energy Department and LLNL spokespeople told AFP they could not comment or provide confirmation regarding the FT report, but said US Energy Secretary Jennifer Granholm would "announce a major scientific breakthrough" on Tuesday.

The LLNL spokesperson added that their "analysis is still ongoing."

"We look forward to sharing more on Tuesday when that process is complete," she said.

The fusion reaction that produced a 120 percent net energy gain occurred in the past two weeks, the FT said, citing three people with knowledge of the preliminary results.

The Washington Post later reported two people familiar with the research confirmed the development, with a senior fusion scientist telling the newspaper, "To most of us, this was only a matter of time."

Nuclear fusion is considered by some scientists to be a potential energy of the future, particularly as it produces little waste and no greenhouse gases.

"If this fusion energy breakthrough is true, it could be a game changer for the world," tweeted Ted Lieu, a member of Congress from California.

Fusion differs from fission, the technique currently used in nuclear power plants, by fusing two atomic nuclei instead of splitting one.

The LLNL fusion facility consists of almost 200 lasers the size of three football fields, which bombard a tiny spot with high levels of energy to initiate a fusion reaction.