Fire crews were fighting a new blaze at reactor number four at the quake-hit Fukushima No. 1 nuclear power plant, operator Tokyo Electric Power (TEPCO) said.

"We are battling the fire now," a spokesman said. The government later said the fire was under control.

Explosions and an earlier fire at the plant had unleashed dangerous levels of radiation on Tuesday, sparking a collapse on the stock market and panic buying in supermarkets.

The prospect of a meltdown sent stocks and commodity prices plunging around the globe, as markets feared the crippling effects on the world's third-largest economy.

In Japanese towns and cities, fearful citizens stripped shelves of food and water, prompting the government to warn that panic buying could hurt its ability to provide aid to areas devastated by Friday's massive quake and tsunami.

But scared Tokyo residents filled outbound trains and rushed to shops to stock up on face masks and emergency supplies amid heightening fears of radiation headed their way.

Radiation levels around the Fukushima No.1 plant on the eastern coast had "risen considerably", Prime Minister Naoto Kan said, and his chief spokesman announced it had reached the point where it endangered human health.

In Tokyo, 250 kilometres (155 miles) to the southwest, authorities also said that higher-than-normal but not harmful radiation levels had been detected in the capital, the world's biggest urban area.

Kan warned people living up to 10 kilometres beyond a 20-kilometre exclusion zone around the nuclear plant to stay indoors.

"I would like to ask the nation, although this incident is of great concern, I ask you to react very calmly," he said.

The fire broke out in the plant's number-four reactor, meaning that four of the facility's six reactors were in trouble -- and temperatures were reportedly rising in the other two.

Radiation levels later dropped at both the plant and in Tokyo, chief government spokesman Yukio Edano said.

As crews battled to avert a nuclear disaster, TEPCO said it may pour water from helicopters to stop fuel rods from being exposed to the air and releasing even more radioactivity.

The UN weather agency said winds were currently blowing radioactive material towards the ocean, and that there were "no implications" for Japan or nearby countries.

President Barack Obama said he was "deeply worried" about the potential human cost of the crisis and vowed to "further improve" the safety of US atomic facilities. The White House however said it was not recommending that Americans leave Tokyo.

France's Nuclear Safety Authority said the disaster now equated to a six on the seven-point international scale for nuclear accidents, ranking the crisis second only in gravity to Chernobyl.

Europe's energy commissioner Guenther Oettinger went further and dubbed the nuclear disaster an "apocalypse", saying Tokyo had almost lost control of events at the Fukushima plant.

"There is talk of an apocalypse and I think the word is particularly well chosen," he said in remarks to the European Parliament.

Japan however maintained its assessment of the disaster at a four, placing it behind the 1979 Three Mile Island accident in the United States.

The disaster prompted European Union nations to agree to conduct safety "stress tests" on their scores of nuclear power plants housing a total of 143 reactors.

On top of the atomic emergency, Japan is struggling to cope with the enormity of the damage from the record quake and the tsunami that raced across vast tracts of its northeast, destroying everything in its path.

The official death toll rose to 3,373 confirmed dead, police said, with the number of those unaccounted for at 6,746. Officials have said at least 10,000 were likely to have perished.

In the only country in the world to have experienced a nuclear attack -- two bombs dropped by the United States during World War II killed some 200,000 people -- Japanese citizens are gripped by dread of nuclear catastrophe.

"What we most fear is a radiation leak from the nuclear plant," Kaoru Hashimoto, 36, a housewife living in Fukushima city, 80 kilometres northwest of the stricken plant, told AFP by phone.

Hashimoto said supermarkets were open but shelves were bare. "Many children are sick in this cold weather, but pharmacies are closed. Emergency relief goods have not reached evacuation centres in the city.

"Everyone is anxious and wants to get out of town, but there is no more petrol," she said.

More than 200,000 people have already been evacuated from the exclusion zone around the crippled plant.

The crisis at the ageing Fukushima nuclear plant has worsened daily since Friday's quake and tsunami knocked out cooling systems.

Explosions hit the buildings housing reactors one and three Saturday and Monday. On Tuesday, a blast hit reactor two at the plant and there was also an explosion at reactor four which started a fire.

The UN's nuclear watchdog, the International Atomic Energy Agency, said Tokyo had asked for expert assistance in the aftermath of the quake, which US seismologists are now measuring at magnitude 9.0, revised up from 8.9.

The IAEA's Japanese chief Yukiya Amano said Tuesday there might be limited core damage at the second reactor but repeated that he did not think the situation could escalate to rival the world's worst nuclear accident at Chernobyl in Ukraine in 1986.

"I continue to think that the Chernobyl and Fukushima reactors are different," he said, reiterating that unlike Chernobyl, Fukushima had a primary containment vessel, and the reactor had shut down automatically when the earthquake hit, so there was no chain reaction going on.

The devastation in tsunami-hit areas such as Sendai city in the northeast, however, is absolute.

At the once-bustling regional airport, small planes jutted out at awkward angles from thick mud amid the wreckage of clusters of wooden beachfront houses that were splintered into flotsam in an instant by the waves.

The machinery of modern life has been crumpled almost beyond recognition as far as the eye can see -- cars are stuck incongruously into the few remaining structures or balanced on top of wrecked homes.

Aid workers and search teams from across the world have joined 100,000 Japanese soldiers in a massive relief push in the shattered areas.

Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless.

And with nerves still jangling, aftershocks continued to hit the country, while a strong and shallow earthquake of 6.0 magnitude rattled Tokyo late Tuesday -- the epicentre was to the southwest, in the opposite direction from the capital to that of Friday's massive quake.

The government expects a "considerable" economic impact from the disaster, which has plunged the nation into what Prime Minister Kan called its worst crisis since World War II.

Singapore's DBS Bank estimated that the twin disasters would cost Japan's economy about $100 billion, or about two percentage points of its annual gross domestic product.

burs/mtp/ft

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