Alarm over Japan’s nuclear disaster grew with more foreign governments advising their citizens to flee Tokyo as army helicopters dumped water on an overheating plant at the centre of the crisis.

Six days after a massive earthquake and tsunami plunged Japan into its worst crisis since World War II, the United States and Britain chartered flights for nationals trying to leave and China moved thousands of citizens to Tokyo for evacuation.

Commercial airline tickets were scarce and some companies hired private jets to evacuate staff. In Tokyo the streets were quiet but calm as the Japanese people, though deeply concerned, mostly remained stoic over the emergency.

At the stricken Fukushima No. 1 plant, 250 kilometres from Tokyo, Chinook military helicopters dumped tonnes of water in a desperate bid to cool reactors crippled by the earthquake to prevent a catastrophic meltdown.

Fire engines were put into action to douse fuel rods inside reactors and containment pools submerged under water to stop them from degrading due to exposure to the air and emitting dangerous radioactive material.

“Based on what experts have told us, it’s important to have a certain level of water (in the pools) before we can start to see any positive effect,” chief government spokesman Yukio Edano told reporters.

The official toll of the dead and missing from the twin disasters, which pulverised the northeast coast, now approached 15,000, police said, as aftershocks continued to rattle a jittery nation.

The number of confirmed dead rose to 5,692, with more than 80,000 buildings damaged and 4,798 destroyed.

But as Japanese and international teams mounted a massive search and relief effort, reports from some battered coastal towns suggested the final death toll could be far higher.

Millions of people have been left without water, electricity, fuel or enough food and hundreds of thousands more were homeless, the misery compounded by heavy snowfalls, freezing cold and wet conditions.

A cold snap brought heavy blizzards over the country’s northeast overnight, covering the tsunami-razed region in deep snow, all but extinguishing hopes of finding anyone alive in the debris.

“We’re already seeing families huddling around gas fires for warmth,” said Save the Children’s Steve McDonald.

“In these sorts of temperatures, young children are vulnerable to chest infections and flu,” he added, estimating that the disaster had left 100,000 children homeless.

The tense nation also saw the stock market fall again yesterday, closing down 1.44 per cent on fears about the economic impact – concerns that have also seen global stocks drop.

The latest threat at the Fukushima plant was the fuel-rod pools, which contain used rods that have been withdrawn from reactors yet remain highly radioactive.

They are immersed in cooling water for many years until they shed enough heat to become manageable for storage.

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