Japan's government on Friday rejected calls from a UN rights expert to halt the return of women and children to areas affected by the Fukushima nuclear disaster over radiation fears.

UN special rapporteur Baskut Tuncak on Thursday warned that people felt they were "being forced to return to areas that are unsafe, including those with radiation levels above what the government previously considered safe."

In the wake of the Fukushima disaster, Japan's government lifted its standard for the acceptable level of radiation to 20 millisieverts per year from 1 millisievert.

It has been urged to revise that level back down again, but has rejected calls to do so, a decision Tuncak called "deeply troubling."

"Japan has a duty to prevent and minimise childhood exposure to radiation," he said.

But Japan's government rejected the criticism, saying Tuncak's comments were based on "one-sided information and could fan unnecessary fears about Fukushima," a foreign ministry official told AFP.

Japan's government has gradually lifted evacuation orders on large parts of the areas affected by the disaster, which occurred when a massive tsunami sent reactors at the Fukushima Daiichi plant into meltdown in March 2011.

But other areas remain under evacuation orders because of continued high levels of radiation.

Japan's government has pushed hard to return affected areas to normal, but has faced criticism that what it refers to as "safe" radiation levels are not in line with international standards.

Around 12,000 people who fled their homes for fear of radiation have filed dozens of lawsuits against the government and the Tokyo Electric Power Company (TEPCO), the operator of the stricken nuclear plant.

The Fukushima disaster was the worst since Chernobyl in 1986, though there has only been one death linked to it. More than 18,000 people were killed or left missing in the tsunami that prompted the meltdown.