Japan declares nuclear emergency
Pressure in several reactors is considerably higher than normal, Japan's nuclear agency says
Japanese authorities have declared a state of emergency at two nuclear power plants, after Friday's devastating earthquake caused malfunctions.
Cooling systems inside several reactors at the Fukushima 1 and 2 power plants have stopped working properly, leading to a build-up of pressure.
Thousands of people have been ordered to evacuate the area near the plants.
Technicians are starting to release vapour to lower the pressure in some of the reactors.
Some of the released gases could be radioactive, but officials insisted the procedure would pose 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.
Continue reading the main story AFFECTED POWER PLANTS Onagawa - all three reactors shut down automatically
Fukushima Daiichi - reactors 1,2 and 3 shut down automatically; reactors 4,5 and 6 were not in operation; reactor 1 and one other were not cooling as expected
Fukushima Daini - all four reactors shut down automatically, three experiencing cooling problems
Tokai - single operational reactor shut down automatically
Defence officials said troops trained in chemical disasters had been sent to the plants in case of a radiation leak.
Earlier, the authorities say they are taking wind direction into account when planning the release of radioactive vapour, which they insist 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 fleet 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 reactor core, completing the loop.
In total, the country has 55 reactors providing about one-third of the nation's electricity.
An explosion has been reported at Japan's stricken  nuclear power plant after its cooling system was damaged by a massive 8.9 magnitude earthquake that hit off the North-East coast of Japan on Friday. The explosion occurred at around 3:00pm local time (6:00am UTC). According to the plant's operator, , four workers were injured in the explosion.

The explosion occurred as workers were attempting to cool a reactor by injecting water into its core. Local media are reporting that the plant's exterior walls have been destroyed and only a skeleton structure remains.

More than 45,000 residents within a 10-kilometre radius of the plant have been evacuated.

Television reports show white smoke coming from the plant. Local authorities are warning residents in the vicinity of the plant to stay indoors, turn off air-conditioners and not to drink tap water. According to Japan's nuclear agency, radioactive  and  has been identified at the plant. Authorities speculate that this indicates that containers holding uranium fuel at the site may have ruptured and are leaking.