Japan has declared a state of emergency at two nuclear power plants, as officials try to confirm whether a reactor has gone into meltdown.Cooling systems inside several reactors at the Fukushima 1 and 2 power plants stopped working after Friday's earthquake damaged power supplies.
Huge pressure has been building up and a small amount of radiation has been released from one of the reactors.
Thousands of people have been ordered to evacuate the area near the plants.
Technicians earlier released vapour to lower the pressure in some of the reactors.
Japan's Prime Minister Naoto Kan confirmed that a "minute amount" of radioactive material had escaped from a reactor during the procedure.
Later, officials said it was possible that one of the reactors had gone into meltdown.
Experts say there is a low risk of a wide-scale nuclear disaster because the reactors are light-water units, meaning an explosion is unlikely.
And officials have insisted that the procedure to relieve pressure from the reactor posed no risk to the public.
Radioactivity levels in the control room of the Fukushima 1 plant were reportedly running at 1,000 times normal.The earthquake knocked out the power supply to the plants, which then automatically shut down.
But it seems that back-up generators at Fukushima 1 designed to power the plant's basic functions also failed.
Two reactors at the Fukushima 1 plant were left without sufficient cooling.
And officials later said they were unable to control the cooling systems in three reactors at Fukushima 2 plant, 11km (seven miles) south of Fukushima 1.
Wind factor Under Japanese law, an emergency must be declared if a cooling system fails, if there is a release of radiation or if there is a dangerous level of water in the reactor.
Prime Minister Naoto Kan declared the emergency. He announced the evacuations after visiting the plant.
Defence officials said troops trained in chemical disasters had been sent to the plants in case of a radiation leak.Earlier, the authorities said they were taking wind direction into account when planning the release of radioactive vapour, which they insisted would not be in quantities big enough to affect human health.
"It's possible that radioactive material in the reactor vessel could leak outside but the amount is expected to be small and the wind blowing towards the sea will be considered," Chief Cabinet Secretary Yukio Edano told a news conference.
The six reactors at Fukushima 1 came online in the 1970s; Fukushima 2 has four reactors, built during the 1980s.
All nuclear facilities in Japan are designed to withstand earthquakes.
But experts say there appears to have been multiple failures at the Fukushima facilities, raising serious questions about the safety of Japan's reactors.
The reactors at Fukushima 1 are Boiling Water Reactors (BWR), one of the most commonly used designs, and widely used throughout Japan's array of nuclear power stations.
Heat is produced by a nuclear reaction in the core, causing the water to boil, producing steam. The steam is directly used to drive a turbine, after which it is cooled in a condenser and converted back to water. The water is then pumped back into the reactor core, completing the loop.
In total, Japan has 55 reactors providing about one-third of the nation's electricity.