The developments raised fears of a meltdown at the plant as officials scrambled to contain what could be the worst nuclear disaster since the Chernobyl explosion in 1986 that shocked the world.
The Japanese plant was damaged by Friday's 8.9-magnitude earthquake, which sent a 10-metre (33-foot) tsunami ripping through towns and cities across the northeast coast. Japanese media estimate that at least 1,300 people were killed.
"We are looking into the cause and the situation and we'll make that public when we have further information," Chief Cabinet Secretary Yukio Edano said after confirming the explosion and radiation leak at the plant.
Edano said an evacuation radius of 10 km (6 miles) from the stricken 40-year-old Daiichi 1 reactor plant in Fukushima prefecture was adequate, but an hour later the boundary was extended to 20 km (13 miles). TV footage showed vapour rising from the plant, 240 km (150 miles) north of Tokyo.
Along Japan's northeast coast, rescue workers searched through the rubble of destroyed buildings, cars and boats,
looking for survivors in hardest-hit areas such as the city of Sendai, 300 km (180 miles) northeast of Tokyo.
Dazed residents hoarded water and huddled in makeshift shelters in near-freezing temperatures. Aerial footage showed buildings and trains strewn over mudflats like children's toys.
"All the shops are closed, this is one of the few still open. I came to buy and stock up on diapers, drinking water and food," Kunio Iwatsuki, 68, told Reuters in Mito city, where residents queued outside a damaged supermarket for supplies.
Across the coastline, survivors clambered over nearly impassable roads. In Iwanuma, not far from Sendai, people spelled S.O.S. out on the roof of a hospital surrounded by water, one of many desperate scenes.
The earthquake and tsunami, and now the radiation leak, present Japan's government with its biggest challenge in a generation.
The explosion at Chernobyl's nuclear plant's fourth reactor in 1986 sent thousands of tonnes of toxic nuclear dust billowing across the Ukraine, Russia and Belarus. It was the worst civil nuclear disaster.
The blast at the Japanese nuclear facility came as plant operator Tokyo Electric Power Co (TEPCO) worked desperately to reduce pressures in the core of the reactor.
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