TOKYO - Residents of a Japanese village well-outside of an evacuation zone set around a crippled nuclear power plant have been told to prepare to be evacuated, a local official said on Monday.

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Chief Cabinet Secretary Yukio Edano said on Monday that a 20km (12 miles) evacuation may need to be extended in some areas after data on accumulated radiation is fully assessed.

Extensions to the exclusion zone around the Fukushima Daiichi plant will depend on how much accumulated radiation has been detected in each area, Edano added.

Some 5,000 residents in Iitate village, 40 km from the plant, have already been told to prepare to evacuate, with local media reporting the evacuation may not take place for a month.

"The government appears to have informed the village yesterday (Sunday) about its inclusion in the so-called planned evacuation zone," an official at the village's administrative office told Reuters.

"Apparently it will not be an immediate evacuation taking into consideration the amount of radiation that residents are likely to be exposed to in the long term," he said.

A massive earthquake and tsunami on March 11 left 28,000 people dead or missing in Japan and the country struggling to cope with the worst nuclear crisis since Chernobyl.