Japan has deployed military helicopters, high-pressure water cannons and fire trucks in an increasingly desperate attempt to cool overheating nuclear fuel at the wrecked Fukushima plant.
While the helicopters flew combat-style missions to dump batch after batch of seawater onto a stricken reactor, plant operators said they were close to finishing a new power line that could restore cooling systems at the Dai-ichi complex on the north-east coast.
The top US nuclear regulatory official gave a far bleaker assessment of the crisis than the Japanese, and the US ambassador warned American citizens within 50 miles of the complex to leave the area or at least remain indoors.
The Japanese government said it had no plans to expand its mandatory, 12-mile exclusion zone around the plant, while also urging people within 20 miles to stay inside.
The troubles at the nuclear complex were triggered when last week's 9.0-magnitude earthquake and tsunami knocked out power and destroyed backup generators needed for the reactors' cooling systems.
That added a nuclear crisis on top of twin natural disasters believed to have killed tens of thousands of people and left hundreds of thousands homeless.
Four of the plant's six reactors have faced serious crises involving fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures in the pools used to store spent nuclear fuel. Officials also recently announced that temperatures are rising in the spent fuel pools of the last two reactors.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on the complex's damaged Unit 3 earlier, defence ministry spokeswoman Kazumi Toyama said. The choppers dumped at least four loads on the reactor in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.
The dousing is aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit's cooling pool, where used fuel rods are stored, Ms Toyama said.
US Nuclear Regulatory Commission Chairman Gregory Jaczko said at a congressional hearing in Washington that all the water was gone from that unit's spent fuel pool: "We believe radiation levels are extremely high," he said.