The clean-up at Fukushima after its tsunami-sparked nuclear meltdowns is unlike anything humanity has ever undertaken, Japan's prime minister said Saturday during a tour of the plant.

"The massive work toward decommissioning is an unprecedented challenge in human history," the newly-elected Shinzo Abe said. "Success in the decommissioning will lead to the reconstruction of Fukushima and Japan."

Abe was at the crippled Fukushima Daiichi just days after being sworn in following the electoral triumph of his pro-business Liberal Democratic Party.

The prime minister's trip to the still-ruined site, on which he was accompanied by an AFP journalist, is part of a push by his administration to put a lid on the crisis.

Observers widely expect Japan to restart its nuclear programme on the LDP's watch, despite public concerns that the party was partially responsible for the extent of the catastrophe because of a culture of complicity during its more than five-decade rule.

His government said Thursday it would review a pledge by the previous administration to scrap nuclear power within three decades and would give the green light to plants deemed safe by regulators.

Experts have warned, however, that a number of reactors sit above what could be still-active faults, making them vulnerable to quakes.

Underlining Japan's sometimes precarious position at the meeting point of continental plates, a magnitude 5.1 earthquake hit off the Fukushima coast mid afternoon, US geologists said.

A spokesman for plant operator TEPCO said no abnormalities were reported.

All of Japan's 50 reactors were shuttered for inspections in the aftermath of the March 2011 disaster at Fukushima, where a tsunami swamped cooling systems, causing meltdowns.

Reactors raged out of control for months, spewing radiation over a wide area and forcing tens of thousands of people to evacuate.

Abe's visit comes a year after experts said they had brought the wrecked units under control. However, melted fuel remains inside their cores and full decommissioning will take decades.

Dressed in a protective suit and wearing a face mask, Abe was taken by bus to see two of the damaged reactors.

He thanked workers for their efforts at a time when many Japanese are celebrating New Year with family.

"Decommissioning work is hard work, but it is progressing. We owe it all to you," he told them.

The disaster at Fukushima was the worst nuclear accident since Chernobyl. Efforts to clean up the site in Ukraine continue, more than a quarter of a century after a reactor exploded.

Critics said lax oversight by regulators, who appeared to have the interests of power companies at heart, had exacerbated the impact of Fukushima.

Operator Tokyo Electric Power Company (TEPCO) admitted earlier this year it had not carried out upgrade and repair work because it wanted to avoid alarming people.

No one has ever been arrested or prosecuted for the disaster, but TV Asahi reported Saturday prosecutors had quizzed a former TEPCO vice president over his handling of safety assessments that forecast a large tsunami could hit the plant.

The broadcaster said other officials would be questioned in the New Year.

Before Fukushima, nuclear power generated around a third of Japan's electricity and there was little debate about its merits.

The meltdowns provoked fierce opposition, sending tens of thousands of people onto the streets.

The strong vein of anti-atomic sentiment did not translate into success at the ballot box earlier this month for parties championing an end to nuclear power.

Abe's LDP won a healthy majority, despite being widely viewed as the most pro-nuclear option on offer.

Speaking to press in Kawauchi, a city to which residents have been allowed to return because of falling levels of radiation, Abe was guarded on the future of nuclear power.

He said he wanted "a responsible energy policy" and pledged to boost alternative energy and renewables as his government worked out the best energy mix for Japan.