Alarm over Japan's nuclear disaster grew on Thursday with more foreign governments advising citizens to flee Tokyo as army helicopters dumped water on the 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 US 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 (155 miles) 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 stood at 5,457, 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 are homeless, the misery compounded by heavy snowfalls, freezing cold and wet conditions.
Thick snow covered wreckage littering quake-hit areas, all but extinguishing hopes of finding anyone alive in the debris.
A cold snap brought heavy blizzards over the country's northeast overnight, covering the tsunami-razed region in deep snow and vital highways in ice.
"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 stockmarket fall again Thursday, closing down 1.44 percent 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.
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 power plant in an attempt to reactivate its cooling system.
"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," a spokesman for Plant operator Tokyo Electric Power Co. told AFP.
US President Barack Obama offered to give Japan any support that it needs, in a telephone call with Prime Minister Naoto Kan, the Japanese leader's spokesman said.
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.
"If you're in Tokyo or any of the affected prefectures... we are saying that you should depart," said Australian Foreign Minister Kevin Rudd.
Britain, France, Germany 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.
US officials however warned citizens living within 80 kilometers of the plant to evacuate or seek shelter.
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.
Gregory Jaczko, chair of the US Nuclear Regulatory Commission, warned there was no water left in the spent fuel pool of the plant's number-four reactor, resulting in "extremely high" radiation levels.
The UN's International Atomic Energy Agency chief Yukiya Amano said the situation was "very serious" as he prepared to fly out to see the damage for himself.
Edano, the government spokesman, said radiation levels from the plant posed no immediate health threat outside the 20-kilometre exclusion zone despite slightly elevated levels detected in Tokyo over the past few days.
In Taiwan, authorities said they had detected radioactive particles on 26 air passengers arriving from Japan, while inspectors in South Korea reportedly detected radiation on the coat of a man also coming from Japan.