Japan on Monday said it was to widen the evacuation area around a crippled nuclear plant to include territory outside the current 20-kilometre (12-mile) exclusion zone.

Top government spokesman Yukio Edano said within one month people will be told to leave the village of Katsurao, Namie town and Iitate village as well as part of Kawamata town and Minamisoma city, due to long term health concerns.

"The government has designated for evacuation areas where the radiation exposure level is expected to reach 20 millisieverts per year," Edano said.

Edano, who had earlier warned the government was considering a widening of the area, said a uniform extension of the zone was not appropriate.

"There are some places where cumulative levels of radiation are increasing depending on climate and geographical conditions even outside of the 20-kilometre radius circle," he said.

Edano said the situation at the nuclear plant had not deteriorated and the widening of the zone reflected only concerns over the effect of long-term exposure.

"If you continue living in the areas for a long time, like six months or one year, the cumulative radiation levels may get even higher."

"(This evacuation) will impose enormous burdens on residents of these areas, but they are required to move to other places under this evacuation. It is preferable that they evacuate within about one month."

Environmental pressure group Greenpeace welcomed the move, but said it did not go far enough.

"Greenpeace has been asking the government to immediately evacuate places where radiation is concentrated, such as Iitate village, Kawamata town and Namie town. So we think the government has made the right decision," said Japan Executive Director Junichi Sato.

"The move is one of the minimum things that can be done for now. Our monitoring has shown that radiation levels are relatively high at highly-populated areas such as cities of Fukushima and Koriyama.

"The government needs to inform residents in such places of measures to protect themselves from radiation. As these places are outside the evacuation zone, many residents are not fully protecting themselves from radiation."