High levels of radiation leaked from a crippled nuclear plant in tsunami-ravaged northeastern Japan after a third reactor was rocked by an explosion today and a fourth caught fire.
The government warned 140,000 people nearby to stay indoors to avoid exposure as the four-day-old catastrophe escalated.
Japan has told the UN nuclear watchdog a spent fuel storage pond was on fire at a reactor damaged by the earthquake and radioactivity was being released "directly" into the atmosphere.
Tokyo also reported slightly elevated radiation levels, but officials said the increase was too small to threaten the 39 million people in and around the capital, about 270km away.
Prime Minister Naoto Kan said radiation has spread from four reactors of the Fukushima Dai-ichi nuclear plant in Fukushima state, one of the hardest-hit in Friday's 9.0-magnitude earthquake and the ensuing tsunami that has killed more than 10,000 people, plunged millions into misery and pummelled the world's third-largest economy.
Officials just south of Fukushima reported up to 100 times the normal levels of radiation this morning. While those figures are worrying if there is prolonged exposure, they are far from fatal.
Mr Kan and other officials warned there is danger of more leaks and told people living within 30km of the Fukushima Dai-ichi complex to stay indoors to avoid the possibility of radiation sickness.
"Please do not go outside. Please stay indoors. Please close windows and make your homes airtight," Chief Cabinet Secretary Yukio Edano told residents in the danger zone. "Don't turn on ventilators. Please hang your laundry indoors."
Weather forecasts for Fukushima were for snow and wind from the northeast Tuesday evening, blowing southwest toward Tokyo, then shifting and blowing west out to sea. That's important because it shows which direction a possible nuclear cloud might blow.
The crisis is the worst Japan has faced since the atomic bombing of Hiroshima and Nagasaki during World War Two.
It is also the first time that such a grave nuclear threat has been raised in the world since a nuclear plant in Chernobyl, Ukraine exploded in 1986.
"These are figures that potentially affect health. There is no mistake about that," he said.
Some 70,000 people had already been evacuated from a 20km radius from the Dai-ichi complex and about 140,000 remain in the zone for which the new warning was issued.
Three reactors at the power plant were in critical condition in the wake of Friday's quake and tsunami, after losing their ability to cool down and releasing some radiation. A fourth reactor that was offline caught fire today and more radiation was released, Mr Edano said.
The fire was put out. Even though it was offline, the fourth reactor was believed to be the source of the elevated radiation because of the hydrogen release that triggered the fire.
"It is likely that the level of radiation increased sharply due to a fire at Unit 4," Mr Edano said. "Now we are talking about levels that can damage human health.
"These are readings taken near the area where we believe the releases are happening. Far away, the levels should be lower."
Officials said 50 workers, all of them wearing protective radiation gear, were still trying to put water into the reactors to cool them.
They say 800 other staff were evacuated.
The fires and explosions at the reactors have injured 15 workers and military personnel and exposed up to 190 people to elevated radiation.
In Tokyo, slightly higher-than-normal radiation levels were detected Tuesday but officials insisted there are no health dangers. "The amount is extremely small, and it does not raise health concerns. It will not affect us," an official said.
Japanese government officials are being rightly cautious, said Donald Olander, professor emeritus of nuclear engineering at University of California at Berkeley. He believed even the heavily elevated levels of radiation around Dai-ichi are "not a health hazard". But without knowing specific dose levels, he said it was hard to make judgments on the evacuation orders.
"Right now it's worse than Three Mile Island," Prof Olander said. But, he said, it's nowhere near the levels released during Chernobyl.
The death toll from last week's earthquake and tsunami jumped as police confirmed the number killed had topped 2,400, though that grim news was overshadowed by a deepening nuclear crisis.
Officials have said previously that at least 10,000 people may have died in Miyagi province alone.
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