Sweating inside their plastic protection suits, thousands of men toil in Japan's muggy early summer in a vast effort to scrub radiation from the villages around Fukushima.

The mission is to decontaminate hundreds of square kilometres (miles) that were polluted when reactors went into meltdown after a huge tsunami struck Fukushima Daiichi nuclear plant in March 2011.

No stone is left unturned: diggers scrape away the top layer of earth in fields, school courtyards and around the buildings of villages, while houses, buildings, roads and parking lots are scrubbed clean.

At least 20,000 people -- all dressed in the special gloves, masks and boots required for workers in the nuclear industry -- are involved in the clean-up, according to the environment ministry.

Some 2.5 million black bags filled with contaminated soil, plants and leaves wait at the sites or in one of the nearly 800 temporary outdoor storage facilities that have been set up across the disaster zone.

The mammoth effort comes as Japan's government prepares to declare sections of the evacuation zone habitable again.

That will mean evacuees can return to the homes they abandoned more than four years ago. It will also mean, say campaigners, that some people will have no choice but to go back because it will trigger the ending of some compensation payments.

Government-run decontamination efforts are under way in 11 cities where Tokyo says that at present, anyone living there would be exposed to radiation levels of more than 20 millisieverts (mSv) a year.

The globally-accepted norm for radiation absorption is 1 mSv per year, although the International Atomic Energy Agency (IAEA) and others say anything up to 20 mSv per year poses no immediate danger to human health.

The settlement of Naraha, which lies just 20 kilometres (12 miles) from the plant, is expected to be declared safe in September.

The government intends to lift many evacuation orders by March 2017, if decontamination progresses as it hopes.

Still, the area immediately surrounding the plant remains uninhabitable, and storage sites meant to last 30 years are being built in the villages closest to the complex.

For now, only residential areas are being cleaned in the short-term, and the worst-hit parts of the countryside are being omitted, a recommendation made by the IAEA.

But that strategy has troubled environmentalists, who fear that could lead to re-contamination as woodlands will act as a radiation reservoir, with pollutants washed out by rains.

In a report on decontamination in Iitate, a heavily forested area that lies northwest of the plant, campaign group Greenpeace says these selective efforts will effectively confine returnees to a relatively small area of their old hometowns.

"The Japanese government plans, if implemented, will create an open-air prison of confinement to 'cleaned' houses and roads... and the vast untouched radioactive forests continue to pose a significant risk of recontamination of these 'decontaminated' areas to even higher levels," the report, published Tuesday, says.

Some 39 other municipalities which were not evacuated after the accident, and which have radiation levels deemed safe for humans, are also being decontaminated by local authorities.