At the stricken Fukushima No. 1 plant, 250 kilometres (155 miles) from Tokyo, Chinook military helicopters have dumped tonnes of water in a desperate bid to cool reactors crippled by the earthquake to prevent a catastrophic meltdown.

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

The fuel-rod pools contain used rods that have been withdrawn from reactors yet remain highly radioactive.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.

Water in one of the pools was evaporating because of the rods' heat, and temperatures were slowly rising in two other pools because coolant pumps were knocked out by the March 11 quake and tsunami, experts said.

They warned that if the tanks run dry and leave the fuel rods exposed, the rods could melt or catch fire, creating potentially lethal levels of radiation.

At the same time, Japanese engineers were focused on restoring the power supply to the stricken plant in an attempt to reactivate its cooling system.

The nuclear safety agency said early Friday that plant operator Tokyo Electric Power Company (TEPCO) had managed to get a line from a regional power firm into the plant site.

"But the line has yet to reach the reactors' power system and it will take 10 or 15 hours to connect the line to it," an agency spokesman said.

A TEPCO spokesman earlier told AFP: "If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel."

In an update, the International Atomic Energy Agency said "engineers plan to reconnect power to unit two once the spraying of water on the unit three reactor building is completed".

The UN nuclear watchdog separately said the situation had not worsened "significantly" over the past 24 hours but warned it would be premature to talk about a ray of hope.

As workers battled to avert meltdown, a newspaper reported that Japan had turned down a US offer of assistance in cooling the fuel rods immediately after the damage to the plant.

The government and TEPCO, both having first thought the cooling system could be restored by themselves, rejected the offer as they believed "it was too early to take", the Yomiuri Shimbun said, quoting a senior official of the ruling Democratic Party of Japan.

Some officials had pointed out that Japan could have avoided the crisis if the government had accepted help straight away, the paper said.

US President Barack Obama Thursday offered to give Japan any support that it needs, in a telephone call with Prime Minister Naoto Kan, the Japanese leader's spokesman said.

Paving the way for a more direct role by the US military, the Pentagon said it had sent a team of experts to evaluate what assistance US forces could provide to the plant.

But as crews battled to prevent an atomic disaster, more foreign governments urged their citizens to steer clear of northeast Japan and the capital Tokyo.

Britain, France, Germany, Australia and New Zealand were among the nations advising their nationals to leave Tokyo and shun the northeast region.

The Japanese government has told people living up to 10 kilometres (six miles) beyond a 20-kilometre exclusion zone around the crippled plant to stay indoors. More than 200,000 people have already been cleared from the zone.

The government has said radiation levels from the plant posed no immediate health threat outside the exclusion zone despite slightly elevated levels detected in Tokyo over the past few days.

US officials however warned citizens living within 80 kilometres of the plant to evacuate or seek shelter. The first US charter flight took off for Taiwan carrying almost 100 people, mostly families of US personnel.

The evacuation plans came against a background of mounting concern over the possibility of a nuclear catastrophe.

"The site is effectively out of control," the European Union's energy chief Guenther Oettinger told a European Parliament committee, a day after he said Japan was facing "apocalypse".

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 in gravity only to the level-seven Chernobyl disaster in 1986.

US Energy Secretary Steven Chu said the events in Japan "actually appear to be more serious" than the 1979 accident at Three Mile Island, a partial reactor meltdown that led to small releases of radioactivity.

"To what extent we don't really know now," Chu said in Washington.

IAEA chief Yukiya Amano said the situation was "very serious" before he flew out on Thursday to Japan to see the damage for himself.

A week after the 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.

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.

Despite the magnitude of the disasters, the International Monetary Fund said Japan had the financial resources to cope and had not requested its assistance.

"We believe that the Japanese economy is a strong and wealthy society and the government has the full financial resources to address those needs," IMF spokeswoman Caroline Atkinson told a news conference.

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