Prime Minister Naoto Kan said he wants the plant's owner, the Tokyo Electric Power Company, to produce a timetable for bringing the disaster to an end, "and they will be doing that soon." And a day after his government warned that thousands more people would need to be evacuated from the surrounding region, he pledged to provide jobs, housing and education for those uprooted by the accident.

Japan declared the Fukushima Daiichi crisis a Level 7 event on the international system for rating nuclear accidents on April 12, putting it on par with the 1986 Chernobyl disaster in the former Soviet Union. The top-scale designation was based on the massive release of radioactivity since the accident began, particularly in its early days, and classifies Fukushima Daiichi a "major accident" requiring long-term countermeasures.

Tokyo Electric's president, Masataka Shimizu, issued a new apology for the disaster and the "enormous anxiety" it has caused after the Level 7 designation on April 12.

"We would like to stabilize the situation as soon as possible, and we are working on the measures and steps to cool the reactors and prevent the spread of nuclear substances," he said.

Scientists believe the amount of radiation released is only a tenth of what was released at Chernobyl, said Hidehiko Nishiyama, the chief spokesman for Japan's Nuclear and Industrial Safety Agency. But the levels for radioactive iodine and cesium that have been spewed into the air, water and soil around the plant are in the thousands of trillions of becquerels, 15 times higher than the threshold for a top-scale event, according to figures released by the safety agency.