Updated 4:42 p.m. ET.
Emergency workers frantic to regain control of Japan's dangerously overheated nuclear complex turned to increasingly elaborate methods Thursday to cool nuclear fuel rods at risk of spraying out more radiation.
They tried with police water cannons, heavy-duty firetrucks and military helicopters dropping bucket after enormous bucket of water onto the stricken system.
The International Atomic Energy Agency said a power cable to reactor 2 was in place by 17:30 local time (8:30 GMT) on Thursday, and that engineers plan to reconnect power to the reactor once workers have finished spraying seawater over reactor 3, the BBC reported.
The U.N. nuclear agency warned the situation was "very serious."
Providing water for spent fuel rod pools and reactor cores will be an ongoing focus at the Fukushima Dai-ichi plant, Nuclear Regulatory Commission Chairman Gregory Jaczko said at a press conference Thursday afternoon."This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel cells," he said.
Late Thursday, officials suspended army helicopter flights to dump water on an overheated reactor to evaluate whether it was helping. At the same reactor, soldiers sprayed 30 tons of water toward a pool where used fuel is stored, hoping to cool it and keep it from catching fire. Police failed in an earlier attempt to cool the pool with water cannon.
U.S. and Japanese officials US, Japan differ on danger zone, spent fuel risk gave differing assessments of what was happening at the Fukushima Dai-ichi nuclear plant, 140 miles north of Tokyo. The top U.S. nuclear regulatory official warned of possible high emissions of radiation while the U.S. ambassador urged Americans within 50 miles of the plant on the tsunami-savaged northeastern coast 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.
Acton said that the differing U.S. and Japanese risk assessments are "hard to explain," but that Japan has to consider the fact that with half a million people already homeless, it may not be able handle new evacuees from the nuclear danger zone.
"If they extend the evacuation zone and evacuate more people but they're not able to feed those people or shelter those people, then it's probably better for those people to stay where they are and inside their houses -- which is actually better radiation protection than it might first appear," he said.
Tokyo Electric Power Co., which owns the plant, said it believed workers were making headway in staving off a catastrophe both with the spraying and, especially, with efforts to complete an emergency power line to restart the plant's own electric cooling systems.
"This is a first step toward recovery," said Teruaki Kobayashi, a facilities management official at the power company. He said radiation levels "have somewhat stabilized at their lows" and that some of the spraying had reached its target, with one reactor emitting steam.
"We are doing all we can as we pray for the situation to improve," Kobayashi said. Authorities planned to spray again Friday, and Kobayashi said: "Choices are limited. We just have to stick to what we can do most quickly and efficiently."
In an encouraging development, crews said they were close to finishing a new power line that could restore cooling systems and ease the crisis at the Fukushima plant on the country's northeast coast. The Tokyo Electric Power Company said it expects to restore power to the site on Friday.
But even if power is restored, it's unclear whether the cooling system will be functional given the quake and tsunami damage, nuclear safety expert James Acton told CBS' "The Early Show" Thursday.
The crisis at the nuclear complex was set off when last week's 9.0-magnitude earthquake and tsunami knocked out power and destroyed backup generators needed for the reactors' cooling systems, adding a major nuclear crisis for Japan as it struggled with twin natural disasters that killed more than 10,000 people and left hundreds of thousands homeless.
However, radiation levels in Japan pose no threat to United States or its territories based on "basic physics and basic science," Jaczko said.
During a press conference Thursday afternoon, President Barack Obama also emphasized that experts did not expect harmful levels of radiation to reach U.S. soil and said, "The CDC and public health experts do not recommend that people in the U.S. take precautionary efforts other than staying informed."
Mr. Obama added that in light of the disaster, he is.
The Pentagon is sending an assessment team to Japan to see whether a much larger military unit, called the Chemical Biological Radiation Nuclear and Engineering Consequence Response Force is needed, reports CBS News correspondent David Martin. If the nine-person assessment team decides more forces are needed, it would likely involve several hundred members of the Response Force who are trained in working in a contaminated environment. They would not be involved in shutting down the reactors, simply in decontamination.
They are also sending additional supplies of potassium iodide pills and are airlifting military dependents from Yokohama and Atsugi to Korea.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on the complex's damaged Unit 3 at 9:48 a.m. (8:48 p.m. ET), defense 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.
Watch video shot from a Japanese Self Defense Force helicopter at the Fukushima site, late Thursday, March 16:
Chopper crews flew missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 2,000 gallons of water. Another 9,000 gallons of water were blasted from military trucks with high-pressure sprayers used to extinguish fires at plane crashes, though the vehicles had to stay safely back from areas deemed to have too much radiation.
Special police units with water cannons, however, could not reach the targets from safe distances and had to pull back, said Yasuhiro Hashimoto, a spokesman for the Nuclear And Industrial Safety Agency.
The water drops were aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit's cooling pool, where used fuel rods are stored, Toyama said. The plant's owner, Tokyo Electric Power Co., said earlier that pool was nearly empty, which would cause the rods to overheat and emit even more radiation.
Defense Minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.
U.S. officials, meanwhile, said Unit 4 also was seriously at risk.
Jaczko said at a congressional hearing in Washington Wednesday that all the water was gone from that unit's spent fuel pool. Jaczko said anyone who gets close to the plant could face potentially lethal doses of radiation.
"We believe radiation levels are extremely high," he said.
Japanese officials earlier had expressed similar worries about the fuel pool, but early Wednesday afternoon they said the rods in the pool were believed to be covered. However, with much of the monitoring equipment in the plant inoperable it was impossible to be sure of the situation.
"We haven't been able to get any of the latest data at any spent fuel pools. We don't have latest water levels, temperatures, none of the latest information," Masahisa Otsuki, a Tokyo Electric Power Co. official said Wednesday.
Chief Cabinet Secretary Yukio Edano said that along with the helicopter water drops, special police units would use water cannons -- normally used to quell rioters -- to spray water onto the Unit 4 storage pool, allowing workers to stay farther away.
Emergency workers were forced to temporarily retreat from the plant Wednesday when radiation levels soared, losing precious time. While the levels later dropped, they were still too high to let workers get close.
The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.
A core team of 180 emergency workers has been at the forefront of the struggle at the plant, rotating in and out of the complex to try to reduce their radiation exposure.
But experts said that anyone working close to the reactors was almost certainly being exposed to radiation levels that could, at least, give them much higher cancer risks.
"I don't know any other way to say it, but this is like suicide fighters in a war," said Keiichi Nakagawa, associate professor of the Department of Radiology at University of Tokyo Hospital.
Experts note, though, that radiation levels drop quickly with distance from the complex. While elevated radiation has been detected well outside the evacuation zone, experts say those levels are not dangerous.
U.S. officials were taking no chances, and Prime Minister Naoto Kan and President Obama spoke about the crisis early Thursday.
In a statement, U.S. Ambassador John V. Roos made his evacuation recommendation "in response to the deteriorating situation" at the Fukushima complex. In Washington, the State Department warned U.S. citizens to consider leaving the country, and offered voluntary evacuation to family members and dependents of U.S. personnel in the cities of Tokyo, Yokohama and Nagoya.
The State Deparmtnet said the first evacuation flight of U.S. citizens left Japan and headed for Taipei, Taiwan. Fewer than 100 people were onboard, mostly dependents of U.S. officials, and also some other private citizens.
Undersecretary of State Patrick Kennedy said the U.S. would provide at least one other aircraft on Friday, depending on need.
Japanese officials raised hopes of easing the crisis early Thursday, saying they may be close to bringing power back to the plant and restoring the reactors' cooling systems.
The new power line would revive electric-powered pumps, making it easier for workers to control the high temperatures that may have led to partial meltdowns in three reactors. The company is also trying to repair its existing disabled power line.
Tokyo Electric officials said they hoped to have the power line working later Thursday, and had electricians standing by to connect the power plant.
Nearly a week after the disaster, police said more than 452,000 people were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short. Both victims and aid workers appealed for more help.
More than 5,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000.
"There is enough food, but no fuel or gasoline," said Yuko Niuma, 46, as she stood looking out over Ofunato harbor, where trawlers were flipped on their sides.
The threat of nuclear disaster only added to Japanese misery.
"The anxiety and anger being felt by people in Fukushima have reached a boiling point," the governor of Fukushima prefecture, Yuhei Sato, fumed in an interview with the Japanese television network NHK. He said evacuation preparations were inadequate, saying centers lacked enough hot meals and basic necessities.
The chief of the U.N. nuclear agency, Yukiya Amano, said he would go to Japan to assess what he called a "very serious" situation and urged Tokyo to provide better information to his organization.
Other countries have complained that Japan has been too slow and vague in releasing details about its rapidly evolving crisis at the complex of six reactors along Japan's northeastern coast.