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Radiation levels around a striken nuclear power plant have fallen but remain above safe limits, Japanese officials say, as the country battles to avoid one of the world's worst nuclear disasters.

Explosions and a fire at the Fukushima Daiichi nuclear power plant unleashed dangerous levels of radiation around the facility yesterday, with the Japanese Prime Minister Naoto Kan calling for people living a further 10 kilometres from a 20-kilometre exclusion zone to stay indoors.

Japanese workers, who have been using fire-fighting equipment to pump seawater into the reactors, said they they may pour water from helicopters to stop fuel rods from being exposed to the air and releasing even more radioactivity.

"We have no options other than to pour water from a helicopter, or to spray water from the ground," a spokesman for operator Tokyo Electric Power Co (TEPCO) said on television. "We have to take action tomorrow or the day after."

In the capital Tokyo - 250 kilometres to the south-west and the world's biggest urban area - authorities also said that higher-than-normal, but not harmful, radiation levels were detected in the capital.

Frightened Tokyo residents filled outbound trains and rushed to shops to stock up on face masks and emergency supplies amid heightening fears of radiation heading their way, despite government warnings that panic buying could hurt its ability to provide aid to areas devastated by Friday's earthquake and tsunami.

An 'apocalypse'

Japan has continued to rate its escalating nuclear crisis at four out of a possible seven, behind the 1979 Three Mile Island accident in the United States, which was rated five, and the 1986 Chernobyl accident, with was rated a maximum seven.

But 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 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.

'Is it a crack? Is it a hole? Is it nothing?'

The director general of the International Atomic Energy Agency, the UN's nuclear watchdog, has also said there might be limited core damage at Fukushima Daiichi nuclear power plant's reactor No.2 - calling it "very worrying".

Yukiya Amano said he wanted more timely and detailed information from Japan - his first hint at frustration with the pace of updates from authorities in his home country.

"The problem is very complicated, we do not have all the details of the information so what we can do is limited," Amano told a news conference. "I am trying to further improve the communication."

"Is it a crack? Is it a hole? Is it nothing? That we don't know yet," Amano said.

But he said the pressure in the containment vessel had not fallen.

"If there is a huge damage the pressure should go down."

Japanese media have criticised the government's handling of the disaster and nuclear plant operator Tokyo Electric Power Co (TEPCO) for its failure to provide enough information on the incident.

Mr Amano said the Vienna-based UN agency planned to send a small team of experts to Japan, possibly to help with environmental monitoring.

He spoke as Japan's crisis appeared to escalate when the operators of the Fukushima plant said one of two blasts had blown a hole in the building housing its No.4 reactor, which meant spent nuclear fuel was exposed to the atmosphere.

Amano said a storage pond with almost 800 spent fuel assemblies - which are highly radioactive - caught fire for about two hours before the blaze was extinguished.

Australians in Japan

The Department of Foreign Affairs and Trade said that as of 6.30am (AEDT) today, 144 Australians believed to be in the disaster areas remain unaccounted for.

A spokeswoman said 3230 Australians, with 119 in the affected regions and including those who were not registered with DFAT, have been confirmed as safe.

There have been no reports of Australian victims, the spokeswoman added.

The overall death toll from the natural disaster has risen above 3300, with more than 6700 people still unaccounted for.

Gripped by fear

In the only country in the world to have experienced a nuclear attack - two bombs dropped by the 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 north-west of the stricken plant, said.

Ms 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.

International flights rerouted

Some flights to Japan were halted or rerouted and air travellers were avoiding Tokyo for fear of radiation from an earthquake-stricken nuclear plant.

Deutsche Lufthansa said it was diverting flights away from Tokyo to Osaka and Nagoya, at least until the weekend. It said planes returning from Tokyo on Monday were not contaminated.

Air China said it had canceled flights to Tokyo from Beijing and Shanghai, mainly due to lack of operational capacity at some airports.

US aviation authorities said they were prepared to take action, including rerouting flights to Japan, if the nuclear crisis worsened.

Other governments, including Britain, Italy and the Netherlands, issued travel warnings.

Survivors found four days after quake

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

In a rare piece of good news, rescuers pulled two survivors, an elderly woman and a man, from underneath the rubble, four days after the earthquake, public broadcaster NHK reported.

But millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless and facing harsh conditions with freezing temperatures overnight, with snow and rain forecast.

The devastation in tsunami-hit areas such as Sendai city 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.

with agencies