The head of the UN nuclear watchdog said Monday his agency would keep working to improve safety after the Fukushima crisis, but no atomic plant could be "100 percent" safe from natural disasters.

Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), made the comments before he meets Prime Minister Shinzo Abe later Monday, as Japan moves closer to restarting two reactors despite objections from a nuclear-wary public.

"What's important for our safety is an evolving process -- we need to improve safety continuously without... falling into complacency," he told a press briefing.

"But any natural disaster can happen in any part of the world... there is no 100 percent safety in the real world."

Amano added that "what we can do is to prevent an accident (as much) as humanly possible, and to get prepared for the mitigation of the consequences" of one.

On March 11, 2011, a 9.0-magnitude undersea earthquake sent a massive tsunami barrelling into Japan's Pacific coastline, sweeping away more than 18,000 victims and destroying coastal communities.

The huge waves swamped cooling systems at the Fukushima plant, sparking reactor meltdowns and explosions that spewed radioactive materials across the vast farm region. It was the worst nuclear accident in a generation.

Although no one died as a direct result of the atomic accident, at least 1,656 Fukushima residents died due to complications related to stress and other conditions.

Tens of thousands were forced to evacuate the area and may never be able to return home.

Plant operator Tokyo Electric Power was severely criticised, including claims it had badly underestimated the maximum size of a tsunami hitting the plant.

Japan's commercial nuclear reactors remain offline, with public opinion generally in favour of keeping them off.

But Abe's government wants to switch nuclear power back on as Tokyo's energy bills skyrocket.

The country was forced to turn to pricy imported fossil fuels to plug the energy gap since nuclear once supplied about one-third of Japan's electricity.

Last week the Nuclear Regulation Authority (NRA) said it would conduct safety checks on two reactors in the southwest.

Japanese media have reported they could come online in a matter of months, although the agency must still win local approval.

The NRA was set up after Fukushima to become an independent body removed from the cloistered nuclear sector. It has since set stricter earthquake and tsunami safety guidelines.

Also Monday, the IAEA chief said his organisation was keeping a close eye on developments in the disposal of spent nuclear waste. Finland will have the world's only final repository for spent fuel when it comes online in 2020.

The site meant that "technically, it is possible to deal with some of the issues of spent fuel and high-level waste but... social and political acceptance is very difficult," Amano said.