Radiation from Japan's quake-stricken Fukushima Daiichi nuclear plant has reached harmful levels, the government says.
The warning comes after the plant was rocked by a third blast, which appears to have damaged one of the reactors' containment systems for the first time.
If it is breached, there are fears of more serious radioactive leaks.
Officials have extended the danger zone, warning residents within 30km (18 miles) to evacuate or stay indoors.
The government later said that radiation levels at the plant's main gate had fallen sharply.
The crisis has been prompted by last Friday's 9.0-magnitude quake and tsunami in north-eastern Japan.
On Tuesday morning, reactor 2 became the third to explode in four days at the Fukushima Daiichi plant.
A fire also briefly broke out at reactor 4, and is believed to have caused radioactive leaks.
Reactor 4 had been shut down before the quake for maintenance, but its spent nuclear fuel rods are still stored on the site.
Chief Cabinet Secretary Yukio Edano said they were closely watching the remaining two reactors at the plant, 5 and 6, as they had begun overheating slightly.
He said cooling seawater was being pumped into reactors 1 and 3 - which were returning to normal - and into reactor 2, which remained unstable.
There was a hydrogen blast in reactor 3 on Monday, following another in reactor 1 on Saturday.
Radiation levels in the Japanese capital - 250km (155 miles) away - were reported to be higher than normal, but officials said there were no health dangers.
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