Scientists in the United States announced on Tuesday that they have created the first nuclear fusion reaction that generated more energy than it took to produce, a landmark achievement that could pave the way for an abundant source of clean energy.

"We have taken the first tentative steps toward a clean energy source," said Jill Hruby, national nuclear security administrator of the US Department of Energy.

Researchers at the multibillion-dollar National Ignition Facility of Lawrence Livermore National Laboratory in California have been studying nuclear fusion since 2009, using 192 lasers to create conditions that cause hydrogen atoms to fuse and release vast amounts of energy.

Known as a net energy gain — a key step toward transforming fusion into a practical source of energy — the goal had been elusive because fusion happens at such high temperatures and pressures that it is extremely difficult to control.

But on Dec 5, scientists at the facility achieved "controlled fusion ignition", in which a fuel is ignited and it keeps burning on its own. The reaction's output is more energy than what is pumped in.

What the scientists did last week was use high-powered lasers to create the fusion reaction that mimicked those that power the sun and the stars. They focused enormous energy from the lasers on a miniature capsule and started a reaction that produced about 1.5 times more energy than was contained in the light source used to produce it.

"Ignition is the first step, a truly monumental one that sets the stage for a transformational decade and high energy density science and fusion research," laboratory director Kim Budil said.

The breakthrough will pave the way for advancements in national defense and the future of clean power, said US Energy Secretary Jennifer Granholm.

"Ignition has allowed us to replicate, for the first time, certain conditions that are found only in the stars and the sun," Granholm said.

Granholm told a news conference in Washington that "this milestone moves us one significant step closer to the possibility of zero-carbon abundant fusion energy powering our society".

Though ignition is a milestone in harnessing nuclear fusion, turning it into a commercial power supply will require clearing additional scientific, technical and financial hurdles.

Scientists have said that while fusion energy holds the potential to provide round-the-clock electricity without pollution or radioactive risks of traditional coal, gas and nuclear power plants, it will be decades before any of it is brought to the power grid, if it ever is.

"It's almost like it's a starting gun going off," said Dennis Whyte, director of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology and a leading expert in fusion research. "We should be pushing toward making fusion energy systems available to tackle climate change and energy security," he said.

But Ian Hutchinson, a professor of nuclear science and engineering at MIT, said, "Useful energy production from miniature fusion explosions still faces enormous engineering challenges, and we don't know if those challenges can be overcome."

Agencies contributed to this story.