Some areas within the 20-kilometer evacuation zone around the stricken Fukushima Daiichi nuclear complex may be off limits for years as a result of unsafe levels of radioactive contamination, Japan 's government said Monday.

"Some places may have to be kept off-limits to residents for a long period of time even after clean-up operations are undertaken," Chief Cabinet Secretary Yukio Edano said at a press conference.

His comments followed Friday's announcement that some areas within the evacuation zone actually had higher levels of radioactive contamination than within the plant's compound itself.

In its first detailed survey of the 20-kilometer evacuation zone around the plant, the education ministry said it found spots where annual radiation exposure to residents could reach 200-500 millisieverts. Most such spots were inside a three-kilometer radius.

The discovery came even though Tokyo Electric Power Co. (9501.TO) has reported a sharp drop in radiation levels inside the plant.

It poses fresh challenges to official efforts to allow evacuees to return to their homes around the plant on a permanent basis.

The government had hoped to narrow the evacuation zone gradually after Tepco brings the damaged reactors under control and stops deadly radiation emissions by January.

The education ministry measured radiation levels at 50 locations within the 20-kilometer radius to estimate exposure for people staying there for a year. The results showed exposure could exceed 100 millisieverts in 15 locations, including one where the annual dosage could reach 508 millisieverts.

That compares with an annual limit of 50 millisieverts for workers in a nuclear plant in normal circumstances, and 250 millisieverts in emergency conditions.

The government requires people to evacuate if the cumulative dosage is likely to exceed 20 millisieverts per year.

Tepco has said the level stands at just 0.4 millisievert per year along the boundary of the plant compound, well below the normal limit of one millisievert for ordinary citizens.

"Radiation spreads like a typhoon. The amount of radioactive substance can be small at the eye of the typhoon, but very large outside," said an official with the Nuclear and Industrial Safety Agency, the main nuclear regulator.

Edano said various options are under consideration to help people who may be displaced for a long period of time, including the possible government purchase or rental of their land.

A detailed policy will be decided in consultation with local authorities based on the results of a further radiation survey and decontamination programs, Edano said.

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