A magnitude 7.1 tremor shook buildings in Tokyo and a wide swathe of eastern Japan on April 11, triggering a small tsunami alert. NHK state television said it caused the off-site power supply for two damaged reactors to shut down.

Tokyo Electric Power Co (TEPCO), which operates the plant, said workers had stopped pouring cooling water on reactors No. 1, No. 2 and No. 3 at Fukushima.

Chief Cabinet Secretary Yukio Edano said villages and towns outside the 20 km (12 mile) evacuation zone that have had more accumulated radiation would be evacuated. Children, pregnant women, and hospitalized patients should stay out of some areas 20-30 km from the Fukushima nuclear complex, he added.

The decision to widen the evacuation band around the Fukushima plant was "based on data analysis of accumulated radiation exposure information", Edano told a news conference.

"These new evacuation plans are meant to ensure safety against risks of living there for half a year or one year," he said. There was no need to evacuate immediately, he added.

Japan had resisted extending the zone despite international concerns over radiation spreading from the six damaged reactors at Fukushima, which engineers are still struggling to bring under control after they were wrecked by the 15-meter tsunami.