Japan on Friday said some areas surrounding the Fukushima nuclear power plant that was wrecked last year by a massive tsunami will likely remain permanently off-limits.

Measurements taken between November and January confirm earlier results which show a level of radioactivity of 470 millisierverts per year when the average, under normal conditions, is less than one per year, according to a government report released Friday.

Some of the highest readings were taken in the town of Futaba, to the northwest of the plant wrecked on March 11.

Contamination however did not spread evenly over the town, with some areas hardly affected, the report added.

The government has cordoned off a 20-kilometre (12-mile) area around the plant, in northeast Japan, but is expected to redefine this in line with levels of radioactive contamination.

A final report by the environment ministry, expected in the coming weeks, is expected to declare as permanently off-limits to human habitation any area with contamination of more than 50 millisieverts per year.

The government is expected to pinpoint areas where contamination hovers between one and 20 millisieverts per year which will be thoroughly decontaminated.

"In between" areas are expected to be declared no-go for many years, but decontamination work will take place with a view to allowing repopulation in the long term.