Rescuers from U.S. conduct search operation in Ofunato, Iwate, Northern Japan

Japan's Prime Minister said on Tuesday that radioactive levels had become high around an earthquake-stricken nuclear power plant after an explosion there, and there was a risk of radiation leaking into the atmosphere.

Naoto Kan urged people within 30 km (18 miles) of the facility north of Tokyo to remain indoors and the French embassy in the capital warned in an advisory that a low level of radioactive wind could reach Tokyo within 10 hours.

Tuesday's explosion was the third at the plant since it was damaged in last Friday's massive earthquake and tsunami.

Authorities have been trying to prevent meltdowns in all three of the Fukishima Daiichi complex's nuclear reactors by flooding the chambers with sea water to cool them down.

As concern about the crippling economic impact of the double disaster mounted, Japanese stocks plunged 7.0 per cent to their lowest level in nearly two years, compounding a drop of 7.6 per cent the day before.

The full extent of the destruction wreaked by last Friday's massive quake and tsunami that followed it was still becoming clear, as rescuers combed through the region north of Tokyo where officials say at least 10,000 people were killed.

"It's a scene from hell, absolutely nightmarish," said Patrick Fuller of the International Red Cross Federation from the Northeastern coastal town of Otsuchi. Kan has said Japan is facing its worst crisis since World War Two and, with the financial costs estimated at up to $180 billion, analysts said it could tip the world's third-biggest economy back into recession.

The U.S. Geological Survey upgraded the quake to magnitude 9.0, from 8.9, making it the world's fourth most powerful since 1900. Car makers, shipbuilders and technology companies worldwide scrambled for supplies after the disaster shut factories in Japan and disrupted the global manufacturing chain.

'Not Chernobyl'

The fear at the Fukushima complex, 240 km (150 miles) north of Tokyo, is of a major radiation leak after the quake and tsunami knocked out cooling systems. The complex had already seen explosions at its No. 1 and No.3 reactors.

Jiji news agency said on Tuesday's explosion had damaged the roof and steam was rising from the complex. It also reported some workers had been told to leave the plant, a development one expert had warned beforehand could signal a worsening stage for the crisis.

The worst nuclear accident since the Chernobyl disaster in Ukraine in 1986 has drawn criticism that authorities were ill-prepared and revived debate in many countries about the safety of atomic power.

Switzerland put on hold some approvals for nuclear power plants and Germany said it was scrapping a plan to extend the life of its nuclear power stations. The White House said U.S. President Barack Obama remained committed to nuclear energy.

Whilst the Fukuskima plant's No.1 and No.3 reactors both suffered partial fuel rod meltdowns, operator Tokyo Electric Power Co (TEPCO) had earlier said the No. 2 reactor was now the biggest concern.

A sudden drop in cooling water levels when a pump ran out of fuel had fully exposed the fuel rods for a time, an official said. This could lead to the rods melting down and a possible radioactive leak.

TEPCO had resumed pumping sea water into the reactor early on Tuesday.

"This is nothing like a Chernobyl," Murray Jennex, a nuclear expert at San Diego State University, said earlier. "At Chernobyl you had no containment structure -- when it blew, it blew everything straight out into the atmosphere."

An explosion at the Soviet Chernobyl plant sent radioactive fallout across northern Europe. U.S. warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation.

The U.S. Seventh Fleet described the move as precautionary. South Korea, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation. France's ASN nuclear safety authority said the accident could be classified as a level 5 or 6 on the international scale of 1 to 7, putting it on a par with the 1979 U.S. Three Mile Island meltdown, higher than the Japanese authorities' rating.

Japan's nuclear safety agency has rated the incidents in the No.1 and No.3 reactors as a 4, but has not yet rated the No. 2 reactor.

Towns flattened

About 850,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.5 million households lack running water.

Tens of thousands of people were missing.

"The situation here is just beyond belief, almost everything has been flattened," said the Red Cross's Fuller in Otsuchi, a town all-but obliterated. "The government is saying that 9,500 people, more than half of the population, could have died and I do fear the worst." Kyodo news agency reported that 2,000 bodies had been found on Monday in two coastal towns alone.

Whole villages and towns have been wiped off the map by Friday's wall of water, triggering an international humanitarian effort of epic proportions. "When the tsunami struck, I was trying to evacuate people. I looked back, and then it was like the computer graphics scene I've seen from the movie Armageddon. I thought it was a dream . it was really like the end of the world," said Tsutomu Sato, 46, in Rikuzantakata, a town on the northeast coast.

In Tokyo, commuter trains shut down and trucks were unable to make deliveries as supermarket shelves ran empty. Estimates of the economic impact are only now starting to emerge. Hiromichi Shirakawa, chief economist for Japan at Credit Suisse, said in a note to clients that the economic loss will likely be around 14-15 trillion yen ($171-183 billion) just to the region hit by the quake and tsunami.

Even that would put it above the commonly accepted cost of the 1995 Kobe quake which killed 6,000 people. The earthquake has forced many firms to suspend production and global companies -- from semiconductor makers to shipbuilders -- face disruptions to operations after the quake and tsunami destroyed vital infrastructure, damaged ports and knocked out factories.

"The earthquake could have great implications on the global economic front," said Andre Bakhos, director of market analytics at Lec Securities in New York. "If you shut down Japan, there could be a global recession."