Key details:

• Crews work to restore electricity to crippled plant
• Prime minister urges nation to unite
• Hundreds of thousands homeless
• Japan raises severity of crisis
• Radiation found in milk, spinach

TOKYO — Emergency workers racing to cool dangerously overheated uranium fuel scrambled Saturday to connect Japan's crippled reactors to a new power line, with electricians fighting tsunami-shattered equipment to restart the complex's cooling systems.

Though the power line reached the complex Friday, making the final link without setting off a spark and potentially an explosion means methodically working through badly damaged and deeply complex electrical systems at the Fukushima Dai-ichi plant on Japan's northeast coast.

"Most of the motors and switchboards were submerged by the tsunami and they cannot be used," said Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency.

Backup power systems at the Fukushima Dai-ichi plant had been improperly protected, said Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency, leaving them vulnerable to the tsunami that savaged the northeastern coast on March 11 and set off the nuclear emergency.

Meanwhile, officials said radiation above levels considered safe had been found in milk and spinach.

"It's not like if you ate it right away you would be harmed," Chief Cabinet Secretary Yukio Edano said. "It would not be good to continue to eat it for some time."

The failure of Fukushima's backup power systems, which were supposed to keep cooling systems going in the aftermath of the massive 9.0-magnitude earthquake, let uranium fuel overheat and were a "main cause" of the crisis, Nishiyama said.

"I cannot say whether it was a human error, but we should examine the case closely," he told reporters.

A spokesman for Tokyo Electric Power Co., which owns and runs the plants, said that while the generators themselves were not directly exposed to the waves, some of the electrical support equipment was outside. The complex was designed to protect against tsunamis of up to 5 meters (16 feet), he said. Media reports say the tsunami was at least 6 meters (20 feet) high when it struck Fukushima.

Motoyasu Tamaki also acknowledged that the complex was old, and might not have been as well-equipped as newer facilities.

Operators of the plant, which have prompted global worries of radiation leaks, hope to have power reconnected to four of the complex's six units on Saturday, and another on Sunday. However, 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.

Underlining authorities' desperation, fire trucks sprayed water overnight in a crude tactic to cool reactor No. 3, considered the most critical because of its use of mixed oxides, or mox, containing both uranium and highly toxic plutonium.

An initial report Saturday that a survivor of Japan's powerful earthquake and tsunami had been rescued from the rubble of a house in Kesennuma city in northern Japan eight days after the disaster turned out to be false, Kyodo news agency reported. The man in fact had been to an evacuation center already and returned to his ruined home when he was discovered by rescue workers, Kyodo reported.

Japan has raised the severity rating of the nuclear crisis to level 5 from 4 on the seven-level International Nuclear Event Scale, putting it on a par with the Three Mile Island accident in 1979, although some experts say it is more serious. The 1986 Chernobyl disaster in Ukraine, the world's worst nuclear accident, was a 7 on that scale.

The State Department late Friday expanded the area for voluntary evacuations for family members of U.S. personnel in Japan to 13 more prefectures. A warning Wednesday named Tokyo, Nagoya and Yokahama. The warning also authorized departure for family members at Misawa Air Base in northern Japan

Thousands dead, missing and suffering
The operation to avert large-scale radiation has overshadowed the humanitarian aftermath of the 9.0-magnitude quake and 33-foot tsunami that struck on March 11. Japan's police agency said nearly 7,200 are dead and more than 10,900 are missing.

Some of the missing may have been out of the region at the time of the disaster. In addition, the massive power of the tsunami likely sucked many people out to sea.

The Japanese government Friday welcomed ever-growing help from the United States in hopes of preventing a complete meltdown.

"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 on Friday.

Later, Prime Minister Naoto Kan urged the nation to unite.

"We will rebuild Japan from scratch. We must all share this resolve," he said in a nationally televised address, calling the crises a "great test for the Japanese people."

As day broke Saturday, steam rose from Unit 3, an unwelcome development that signaled continuing problems. Emergency crews faced two continuing challenges: 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.

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.

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.

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 U.S. conducted drone flights over the reactor site, strapping sophisticated pods onto aircraft to measure radiation aloft.

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 and residents observed a moment of silence on Friday to mark one week since the prosperous country was stricken.

Nearly 7,000 people have been confirmed killed in the double natural disaster, which turned whole towns into waterlogged and debris-shrouded wastelands. Another 10,700 people are missing with many feared dead.

Some 390,000 people, including many among Japan's aging population, are homeless and battling near-freezing temperatures in shelters in northeastern coastal areas.

Food, water, medicine and heating fuel are in short supply.

"Everything is gone, including money," said Tsukasa Sato, a 74-year-old barber with a heart condition, as he warmed his hands in front of a stove at a shelter for the homeless.

Health officials and the U.N. atomic watchdog have said radiation levels in the capital Tokyo were not harmful. But the city has seen an exodus of tourists, expatriates and many Japanese, who fear a blast of radioactive material.

"I'm leaving because my parents are terrified. I personally think this will turn out to be the biggest paper tiger the world has ever seen," said Luke Ridley, 23, from London as he sat at Narita international airport using his laptop.

"I'll probably come back in about a month."

Though there has been alarm around the world, experts have been warning there is little risk of radiation at dangerous levels spreading to other nations.

The U.S. government said "minuscule" amounts of radiation were detected in California consistent with a release from Japan's damaged facility, but there were no levels of concern.

Amid their distress, Japanese were proud of the 279 nuclear plant workers toiling in the wreckage, wearing masks, goggles and protective suits sealed by duct tape.

"My eyes well with tears at the thought of the work they are doing," Kazuya Aoki, a safety official at Japan's Nuclear and Industrial Safety Agency, told Reuters.

The plight of the homeless also worsened following a cold snap that brought heavy snow to the worst-affected areas.

Nearly 290,000 households in the north were still without electricity, officials said, and the government said about 940,000 households lacked running water.

Aid groups say most victims are getting help, but there are pockets of acute suffering.

"We've seen children suffering with the cold, and lacking really basic items like food and clean water," Stephen McDonald of Save the Children said in a statement on Friday.