Reports from Japan say radioactivity in water at reactor 2 at the damaged Fukushima nuclear plant is 10 million times the usual level.
Workers trying to cool the reactor core to avoid a meltdown have been evacuated, Reuters news agency says.
Earlier, Japan's nuclear agency that levels of radioactive iodine in the sea near the plant have risen to 1,850 times the usual level.
The UN's nuclear agency has warned the crisis could go on for months.
It is believed the radiation at Fukushima is coming from one of the reactors, but a specific leak has not been identified.
Leaking water at reactor 2 has been measured at 1,000 millisieverts/hour - 10 million times higher than when the plant is operating normally.
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