Japanese military helicopters dumped water from huge buckets onto the stricken Fukushima nuclear power plant in a bid to douse fuel rods, television images showed.
A total of three twin-rotor CH-47 Chinooks of the Self-Defence Forces each emptied more than seven tonnes of water onto reactors three and four, according to public broadcaster NHK.
Japanese engineers are focusing their efforts on restoring the power supply to a quake-damaged nuclear plant in an attempt to reactivate its cooling system and avert a meltdown.
The massive earthquake and tsunami knocked out the power supply and back-up generators at the Fukushima No. 1 plant on the Pacific coast, some 250 kilometres (155 miles) northeast of Tokyo.
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The lack of power has sent temperatures soaring in reactors with fuel rods being exposed as the cooling water evaporated and emitting hydrogen gas and possibly radioactive material, triggering fears of a meltdown.
Plant operator Tokyo Electric Power Co. (TEPCO) said it was preparing to restore outside power lines and connect its damaged transmission system to unaffected lines.
"At the moment, we are concentrating our efforts on this work," spokesman Naohiro Omura 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."
The Japanese crews grappling with the world's worst nuclear incident since Chernobyl in 1986 were briefly evacuated after a spike in radiation levels at the plant.
Earlier they contended with a new fire and feared damage to the vessel containing one of the plant's six reactor cores.
There are also major fears about pools holding spent fuel rods at the plant, which need water to keep them cool. Unlike the reactors, they have no containment vessels.
Some 70 workers have been using low-capacity fire pumps to pour seawater to cool reactors at the plant, according to media reports. They are using electricity from borrowed power supply cars.
A Japanese army helicopter aborted its mission to dump water onto the fuel rods due to the high level of radiation above the reactors.
UN atomic watchdog chief Yukiya Amano said the situation at the plant was "very serious" as he prepared to fly out to see the damage for himself.
US warns of 'extremely high' radiation levels
Gregory Jaczko, chair of the US Nuclear Regulatory Commission, warned there was no water left in the spent fuel pool of reactor 4, resulting in "extremely high" radiation levels.
The US military will send a spy drone to take a closer look at the reactors in the troubled plant, Kyodo News reported.
Engineers have been desperately battling a feared meltdown at the 40-year-old plant since the earthquake and tsunami knocked out cooling systems and fuel rods began overheating.
The US embassy in Tokyo warned American citizens living within 50 miles (80 kilometres) of the plant to evacuate or seek shelter.
Other foreign governments also urged their citizens to steer clear of the quake-stricken northeast of Japan and the capital Tokyo amid fears of further aftershocks and a widening nuclear disaster.
Japan's chief government spokesman Yukio Edano however said radiation levels from the plant posed no immediate health threat outside the 20-kilometre exclusion zone.
But as crews battled to prevent a nuclear meltdown, the European Union's energy chief said the situation had spun out of control.
"The site is effectively out of control," energy commissioner Guenther Oettinger told a European Parliament committee, one 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 only in gravity to the level-seven Chernobyl disaster in 1986.
Amano, the Japanese chief of the International Atomic Energy Agency, had insisted there was no comparison to the Chernobyl crisis, when radiation spewed across Europe.
