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 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 the plant's 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.

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.

In a televised address, Prime Minister Naoto Kan said: "There is still a very high risk of more radiation coming out."

He said that those living within between 20km (12 mile) and 30km of the plant were at risk and should not leave their homes.

Residents within 20km have already been advised to evacuate, and the premier said anyone left in that exclusion zone must leave.

Japan also announced a 30-km no-fly zone around the reactors to prevent planes spreading the radiation further afield.