Huge blast at Japan nuclear power plant
A massive explosion has struck a Japanese nuclear power plant after Friday's devastating earthquake.
A huge pall of smoke was seen coming from the plant at Fukushima and several workers were injured.
Japanese officials fear a meltdown at one of the plant's reactors after radioactive material was detected outside it.
A huge relief operation is under way after the 8.9-magnitude earthquake and tsunami, which killed more than 600.
Hundreds more people are missing and it is feared about 1,300 may have died.
The offshore earthquake triggered a tsunami which wreaked havoc on Japan's north-east coast, sweeping far inland and devastating a number of towns and villages.
The term "meltdown" raises associations with two nuclear accidents in living memory: Three Mile Island in the US in 1979, and Chernobyl in Ukraine in 1986.
In both, excess heat in the reactor caused the metallic core to melt. The question is whether the same thing has happened in Fukushima.
It appears that the reactor was shut down well before any melting occurred, which should reduce considerably the risk of radioactive materials entering the environment.
However, the detection of caesium isotopes outside the power station buildings could imply that the core has been exposed to the air.
Although Japan has a long and largely successful nuclear power programme, officials have been less than honest about some incidents in the past, meaning that official re-assurances are unlikely to convince everyone this time round.
Japan's Prime Minister Naoto Kan declared a state of emergency at the Fukushima 1 and 2 power plants as engineers try to confirm whether a reactor at one of the stations has gone into meltdown.
It is an automatic procedure after nuclear reactors shut down in the event of an earthquake, allowing officials to take rapid action.Cooling system failure
Television pictures showed a massive blast at one of the buildings of the Fukushima 1 plant, about 250km (160 miles) north-east of Tokyo.
A huge cloud of smoke billows out and large bits of debris are flung far from the building.
Japan's NHK TV showed before and after pictures of the plant. They appeared to show that the outer structure of one of four buildings at the plant had collapsed after the explosion.
The Tokyo Electric Power Co, the plant's operator, said four workers had been injured.
It is not yet clear in exactly what part of the plant the explosion occurred or what caused it.
Chief Cabinet Secretary Yukio Edano said experts were trying to determine the level of radiation at the site.
Japan's nuclear agency said on Saturday that radioactive caesium and iodine had been detected near the number one reactor of the Fukushima 1 plant.
The agency said this may indicate that containers of uranium fuel inside the reactor may have begun melting.
Air and steam, with some level of radioactivity, has been released from several of the reactors at both plants in an effort to relieve the huge amount of pressure building up inside.
Mr Kan said the amount of radiation released was "tiny".
Thousands of people have been ordered to evacuate the area within a 10-km radius of the plant. BBC correspondent Nick Ravenscroft said police stopped him 60km from the Fukushima 1 plant.
Nuclear reactors at four power plants in the earthquake-struck zone automatically shut down on Friday.
In some of the reactors at the two Fukushima plants the cooling systems, which should keep operating on emergency power supplies, failed.
Without cooling, the temperature in the reactor core builds, with the risk that it could melt through its container into the building housing the system.
Pressure also builds in the containers housing the reactor.
Analysts say a meltdown would not necessarily lead to a major disaster because light-water reactors would not explode even if they overheated.
But Walt Patterson, of the London research institute Chatham House, said "this is starting to look a lot like Chernobyl".
He said it was too early to tell if the explosion's aftermath would result in the same extreme level of radioactive contamination that occurred at Chernobyl.
The explosion was most likely caused by melting fuel coming into contact with water, he told the BBC.
The 8.9-magnitude tremor struck in the afternoon local time on Friday off the coast of Honshu island at a depth of about 24km, 400km (250 miles) north-east of Tokyo.
It was nearly 8,000 times stronger than last month's quake in New Zealand that devastated the city of Christchurch, scientists said.
Some of the same search and rescue teams from around the world that helped in that disaster are now on their way to Japan.