Japan's nuclear watchdog said Wednesday that two atomic reactors were safe enough to switch back on, marking a big step towards restarting the country's nuclear plants which were shuttered after the Fukushima crisis.

But fresh protests -- and accusations that the regulator is a puppet of the powerful atomic industry -- have highlighted the challenges Prime Minister Shinzo Abe faces in bringing back a technology that many Japanese have forever sworn off.

Abe has been trying to persuade a wary public that the world's third largest economy must return to an energy source which once supplied more than a quarter of its power.

Widespread anti-nuclear sentiment has simmered in Japan ever since an earthquake and tsunami in March 2011 caused meltdowns at the Fukushima power plant -- the worst atomic disaster since Chernobyl.

Nuclear Regulatory Authority (NRA) officials Wednesday issued a more than 400 page safety report on the Sendai plant in southern Japan, technically giving the operator the green light to switch on its two reactors -- in what would be the first restart since Japan ushered in tougher regulations last year.

But any restart was unlikely before autumn at earliest, following a month-long public consultation period and the need to win over communities near the plant.

"This is a step forward," Abe said. "I will work towards restarting the plant while getting the understanding of local people."

Business groups have backed Abe's push to bring nuclear power plants back online after Japan's energy bills soared when it was forced to turn to pricey fossil fuels.

Some of the country's utilities -- including Sendai's operator Kyushu Electric Power -- have received billions of dollars in bailout money to rescue their finances which suffered when the plants went offline.

NRA chairman Shunichi Tanaka said the Sendai plant would have to operate under some of the world's toughest safety standards to reduce the risk of another major accident.

"But it is often misunderstood when we talk about safety... we can never say there is zero risk," he told reporters in Tokyo.

- 'Controversial decisions' -

At a public meeting to finalise their decision, Tanaka and his colleagues were met with shouts of "Shame on you!" from a small band of protesters, while demonstrators also gathered outside the Sendai plant.

"The NRA has yielded to the enormous pressure of the nuclear industry and the Abe government... instead of putting the safety of people first," said Kazue Suzuki from Greenpeace Japan.

Following the meltdowns at Fukushima, the country's nuclear reactors were switched off. Two reactors were briefly restarted last year but all of Japan's nuclear plants are currently offline.

Complicating matters, there was no clear roadmap on who would make the final decision to restart reactors, especially if there was strong local opposition.

"It's a problem that the decision-making system is not clear," said Tomoaki Iwai, a politics professor at Nihon University.

"Because no one wants to take responsibility for a controversial decision like this, the prime minister will probably make the final call."

Abe is facing opposition, with one local assembly calling for the Sendai site to be decommissioned, while an anti-nuclear politician won a tight election at the weekend.

- Battle lines drawn -

Former parliamentarian Taizo Mikazuki, 43, narrowly won Sunday to become governor of Shiga prefecture, beating a candidate backed by Abe's ruling Liberal Democratic Party.

The region borders Fukui prefecture, host to 13 idled reactors, and where the battle over nuclear power could see its biggest fight.

Mikazuki has demanded that Tokyo get his approval before any reactor restarts in Fukui.

Japan lies in one of the world's most seismically active areas and is regularly hit by powerful earthquakes.

Worries about whether nuclear plants could withstand another disaster came into focus at the weekend as a strong earthquake struck near Fukushima.

No major damage was reported, but seismologists said the quake was an aftershock of the tremor that sparked 2011's deadly tsunami, and warned of more to come.

A 9.0-magnitude earthquake saw a monster tsunami slam into Japan's Pacific coastline, leaving about 18,000 dead or still missing.

The huge waves swamped the Fukushima plant, sending reactors into meltdown and spewing radioactivity across the adjacent farming region. The area could remain a no-go zone for decades.