The Tokyo Electric Power Co. has connected an external transmission line with the stricken nuclear power plant Friday and confirmed that electricity can be supplied, Reuters reported, in a move hoping to cool down the exposed nuclear fuel rods.

In a statement, the company said it "planned to supply Unit 2 first, followed by Unit 1, Unit 3 and Unit 4 ... because Unit 2 is expected to be less damaged" at the Fukushima Dai-ichi nuclear power plant.

Despite the connection, there is no guarantee the pumps will work even if they have electricity. 

Japan reached out Friday to the U.S. for help in reining in the crisis at its dangerously overheated nuclear complex as the country raised the severity of the accident on-hand.

Japan's nuclear safety agency has raised the rating of the country's nuclear accident from 4 to 5 on a 7-level international scale.

The hallmarks of a level 5 emergency are severe damage to a reactor core, release of large quantities of radiation with a high probability of "significant" public exposure or several deaths from radiation. The alert level applied to reactors No.1, No. 2 and No. 3 at the troubled Fukushima Dai-ichi nuclear plant.

The scale defines a level 4 incident as having local consequences, while a level 5 incident can bring wider consequences.

The U.N.'s atomic energy chief said Friday that the disaster is a race against the clock that demands global cooperation said IAEA chief Yukiya Amano. The official plans to brief his agency's 35-nation board on Monday on the nuclear crisis in Japan after returning from Tokyo.

At the stricken complex, military fire trucks again sprayed the troubled reactor units for a second day, with tons of water arcing over the facility in desperate attempts to prevent the fuel from overheating and spewing dangerous levels of radiation.

"The whole world, not just Japan, is depending on them," Tokyo office worker Norie Igarashi, 44, said of the emergency teams working amid heightened radiation levels at the complex on the northeastern coast.

Last week's 9.0 quake and tsunami set off the nuclear problems by knocking out power to cooling systems at the Fukushima Dai-ichi plant on the northeast coast. Since then, four of the troubled plant's six reactor units have seen fires, explosions or partial meltdowns.

The unfolding crises have led to power shortages in Japan, forced factories to close, sent shockwaves through global manufacturing and triggered a plunge in Japanese stock prices.

"We see it as an extremely serious accident," Yukiya Amano, the head of the U.N.'s International Atomic Energy Agency, told reporters Friday after arriving in Tokyo. "This is not something that just Japan should deal with, and people of the entire world should cooperate with Japan and the people in the disaster areas."

"I think they are racing against the clock," he said of the efforts to cool the complex.

One week after the twin disasters -- which left more than 6,900 dead and another 10,700 missing -- emergency crews are facing two challenges in the nuclear crisis: cooling the reactors where energy is generated, and cooling the adjacent pools where used nuclear fuel rods are stored in water.

Both need water to keep their uranium cool and stop it from emitting radiation, but with radiation levels inside the complex already limiting where workers can go and how long they can remain, it's been difficult to get enough water inside.

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 out radiation.

"Dealing with Unit 3 is our utmost priority," Chief Cabinet Secretary Yukio Edano told reporters.

Edano said Friday that Tokyo is asking the U.S. government for help and that the two are discussing the specifics.

"We are coordinating with the U.S. government as to what the U.S. can provide and what people really need," Edano said.

A U.S. military fire truck was used to help spray water into the crippled Unit 3, according to Air Force Chief of Staff Shigeru Iwasaki, though the vehicle was apparently driven by Japanese workers.

The U.S. vehicle was used alongside six Japanese military fire trucks normally used to extinguish fires at plane crashes.

The fire trucks allowed emergency workers to stay a relatively safe distance from the radiation, firing the water with high-pressure cannons. The firefighters also are able to direct the cannons from inside the vehicle.

Officials shared few details about the Friday operation, which lasted nearly 40 minutes, though Iwasaki said he believed some water had reached its target.

Meanwhile, tsunami survivors observed a minute of silence Friday afternoon at the one-week mark since the 9.0-magnitude quake, which struck at 2:46 p.m. Many were bundled up against the cold at shelters in the disaster zone, pressing their hands together in prayer.

Low levels of radiation have been detected well beyond Tokyo, which is 140 miles 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 holing up in their homes.

The Japanese government has been slow in releasing information on the crisis, even as the troubles have multiplied. In a country where the nuclear industry has a long history of hiding its safety problems, this has left many people -- in Japan and among governments overseas -- confused and anxious.

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 evacuation zone for its citizens, wider than the 30-mile band Japan has ordered.

Crucial to the effort to regain control over the Fukushima plant is laying a new power line to the plant, allowing operators to restore cooling systems. The operator, Tokyo Electric Power Co., missed a deadline late Thursday but hoped to completed the effort late Friday, said nuclear safety agency spokesman Minoru Ohgoda.

But the utility is not sure the cooling systems will still function. If they don't, electricity won't help.
President Barack Obama appeared on television to assure Americans that officials do not expect harmful amounts of radiation to reach the U.S. or its territories. He also said the U.S. was offering Japan any help it could provide.

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 the Fukushima Dai-ichi plant, a core team of 180 emergency workers has been rotating out of the complex to minimize radiation exposure.

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 partial meltdown at Three Mile Island also was ranked a Level 5. The Chernobyl accident of 1986, which killed at least 31 people with radiation sickness, raised long-term cancer rates, and spewed radiation for hundreds of miles (kilometers), was ranked a Level 7.

The Associated Press and NewsCore contributed to this report.