Greenpeace called on Japan on Thursday to evacuate children and pregnant women from a town about 60 kilometres (40 miles) from the stricken Fukushima nuclear plant because of high radiation.

The environmental and anti-nuclear group said its own data from Fukushima town roughly matched that of the government, but that it drew radically different conclusions, especially on the health threat for children.

It demanded Japan's government "provide full financial and logistical support for the prompt evacuation of pregnant women and children living in high-radiation areas and conduct a full clean-up of contaminated areas."

Greenpeace chief Kumi Naidoo, after a visit to a kindergarten in the town where parents have been removing contaminated topsoil, said that Fukushima's people now face both a "radiation catastrophe" and an "information limbo".

Since the March 11 disaster, Japan has raised the legal exposure limit for people, including children, from one to 20 millisieverts per year -- matching the safety standard for nuclear industry workers in many countries.

"This is highly unacceptable," said Jan Beranek, the group's energy campaign leader, at a news conference in Tokyo.

"After the accident in Chernobyl, the Soviet government decided to evacuate anyone living in a place where the annual dose was to exceed five millisieverts.

"In that sense it's really unacceptable and unjustifiable for the Japanese government to insist that 20 millisieverts is harmless."

Beranek said the 20-kilometre evacuation zone around the plant was not wide enough and did not match the irregular pattern of radiation exposure.

In Fukushima town, people, including children, were now being exposed to 10 to 20 millisieverts per year through the air alone -- not counting contaminants inhaled or ingested through dust, water or food -- he said.

Radiation experts agree that children are at highest risk because they are still growing, will have more time to develop cancers and other health defects, and are most likely to develop thyroid cancer.

"At least parts of the population that are sensitive need to be evacuated, and the remaining people who decide to stay for various reasons need to be given proper support and information," Beranek said.

He advised that people avoid breathing in any radioactive dust by wearing face masks or staying indoors at windy times, and that they avoid eating, smoking or drinking when their hands may be contaminated.

Meanwhile, the city of Date, located just outside the nuclear no-go zone, said Thursday it would distribute radiation dosimeters to all its 8,000 pre-school, elementary and junior high pupils, a news report said.

The city said the devices would be part of its efforts to ensure children's health as there were radiation hotspots, the Kyodo News agency said.