The entire world was on alert, watching for any evidence of dangerous spikes in radioactivity spreading from the six-reactor facility, or that damage to the Japanese economy might send ripple effects around the globe.
As day broke in northeastern Japan on Saturday, steam rose from Unit 3, an unwelcome development if not a new one that signaled continuing problems. Emergency crews faced two continuing challenges at the plant: cooling the nuclear fuel in reactors where energy is generated and cooling the adjacent pools where thousands of used nuclear fuel rods are stored in water.
"In hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster," Chief Cabinet Secretary Yukio Edano said Friday.
Crucial to the effort to regain control over the plant is laying a new power line to the complex, allowing operators to restore cooling systems. Tokyo Electric said it has brought the cable to the plant and was expected Saturday to try to connect it to the facility's Unit 2; the utility has already missed a Thursday deadline to do that.
Power company official Teruaki Kobayashi warned that experts will have to check for anything volatile to avoid an explosion when the electricity is turned on. "There may be sparks, so I can't deny the risk," he said.
Even once the power is reconnected, it is not clear if the cooling systems will still work.
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.
The government raised the accident classification for the nuclear crisis from Level 4 to Level 5 on a seven-level international scale. That put it on a par with the Three Mile Island accident in Harrisburg, Pa., in 1979, and signified its consequences went beyond the local area.
Edano also said Tokyo was asking Washington for additional help, yet another change from a few days ago, when Japanese officials disagreed with American assessments of the severity of the problem.
The Science Ministry said radiation levels about 30 kilometers (19 miles) northwest of the Fukushima Dai-ichi plant rose at one time Friday to 0.15 millisieverts per hour, about the amount absorbed in a chest X-ray. While levels fluctuate, radiation at most points at that distance from the facility have been far below that. The ministry did not have an explanation for the rise.
A U.S. military fire truck was among a fleet of Japanese vehicles that sprayed water into Unit 3, according to air force Chief of Staff Shigeru Iwasaki, sending tons of water arcing over the facility in an attempt to prevent nuclear fuel from overheating and emitting dangerous levels of radiation.
Additionally, the United States also conducted overflights of the reactor site, strapping sophisticated pods onto aircraft to measure radiation aloft. Two tests conducted Thursday gave readings that U.S. Deputy Energy Secretary Daniel B. Poneman said reinforced the U.S. recommendation that people stay 50 miles (80 kilometers) away from the Fukushima plant.
American technical experts also are exchanging information with officials from the Tokyo Electric Power Co. which owns the plants, as well as with Japanese government agencies.
Sirens wailed along the devastated northeast coastline on Friday to mark one week since the prosperous country was stricken. Natural forces have claimed the lives of more than 6,900, with many thousands more missing in an area struck first by a magnitude 9.0 earthquake and then an enormous wall of water that seemed to scrape the earth clean.
But amid the misery, the threat of nuclear disaster has riveted international attention.
The tsunami knocked out power to cooling systems at the nuclear plant and its six reactors. In the week since, four have been hit by fires, explosions or partial meltdowns.
The events have led to power shortages and factory closures, hurt global manufacturing and triggered a plunge in Japanese stock prices.
Most of Japan's auto industry is shut down. Factories from Louisiana to Thailand are low on Japanese-made parts. Idled plants are costing companies hundreds of millions of dollars. And U.S. car dealers may not get the cars they order this spring.
Prime Minister Naoto Kan vowed that the disasters would not defeat Japan.
"We will rebuild Japan from scratch," he said in a nationally televised address, comparing the work with the country's emergence as a global power from the wreckage of World War II.
"In our history, this small island nation has made miraculous economic growth thanks to the efforts of all Japanese citizens. That is how Japan was built," he said.
Edano said that the "The unprecedented scale of the earthquake and tsunami that struck Japan, frankly speaking, were among many things that happened that had not been anticipated under our disaster management contingency plans," he said.
"In hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster," he said.
Water in at least one fuel pool - in the complex's Unit 3 - is believed to be dangerously low. Without enough water, the rods may heat further and spew radiation.
While nuclear experts have been saying for days that Japan was underplaying the crisis' severity, Hidehiko Nishiyama of the nuclear safety agency said the rating was raised when officials realized that at least 3 percent of the fuel in three of the complex's reactors had been severely damaged. That suggests those reactor cores have partially melted down and thrown radioactivity into the environment.
Low levels of radiation have been detected well beyond Tokyo, which is 140 miles (220 kilometers) south of the plant, but hazardous levels have been limited to the plant itself. Still, the crisis has forced thousands to evacuate and drained Tokyo's normally vibrant streets of life, its residents either leaving town or staying in their homes.
The Japanese government has been slow in releasing information on the crisis. In a country where the nuclear industry has a long history of hiding safety problems, this has left many people, in Japan and among governments overseas, confused and anxious.
In the disaster zone, tsunami survivors, rescue workers and ordinary people observed a minute of silence Friday at 2:46 p.m. - the moment a week ago when the quake struck. Many were bundled up against the cold. As a siren blared, they lowered their heads and clasped their hands in prayer.
In the largely destroyed town of Hirota, 70-year-old Tetsuko Ito wept as she hugged an old friend she met at a refugee center. One of her sons was missing and another had been evacuated from his home near the Fukushima complex.
Tsunami Relief: Network for Good
"Every day is terrifying. Is there going to be an explosion at the reactor? Is there going to be word my other son is dead?" she said.
She searched for her missing son for three days, then her car ran out of gas.
"I think he's dead. If he was alive, he would have contacted someone, somehow," she said. "My other son is alive, but we don't know if there's going to be a nuclear explosion."
If the situation gets worse in Fukushima, she said her son and his family will have to live at her already crowded house, which escaped the tsunami.
"It's strange when this destroyed area is a place someone would consider safe," she said.
Police said more than 452,000 people made homeless by the quake and tsunami 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 as the chances of finding more survivors dwindled.
About 343,000 Japanese households still do not have electricity and about 1 million have no water.
At times, Japan and the U.S. - two very close allies - have offered starkly differing assessments over the dangers at Fukushima. U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said Thursday that it could take days and "possibly weeks" to get the complex under control. He defended the U.S. decision to recommend a 50-mile (80-kilometer) evacuation zone for its citizens, wider than the 12-mile (20-kilometer) band Japan has ordered.
Talmadge reported from Yamagata, Japan. Associated Press writers Foster Klug in Hirota, Japan, and Elaine Kurtenbach, Tim Sullivan, Shino Yuasa and Jeff Donn in Tokyo contributed to this report.