The head of the U.N.'s nuclear energy agency says Japan is racing against the clock to cool the overheating nuclear reactors at its crippled power plant.

International Atomic Energy Agency head Yukiya Amano gave the critical assessment after flying into Tokyo on Friday and being briefed by Japanese Prime Minister Naoto Kan.

Amano says he underscored his concern about the extreme seriousness of the accident at the Fukushima Dai-ichi nuclear plant.

Smoke billowed from a building at the nuclear power plant Friday as emergency crews worked to reconnect electricity to cooling systems and spray more water on overheating nuclear fuel at the tsunami-ravaged facility.

Four of the troubled Fukushima Dai-ichi plant's six reactor units have seen fires, explosions or partial meltdowns in the week since the tsunami. While the reactor cores where energy is generated are a concern, water in the pools used to store used nuclear fuel are also major worries. Water in at least one fuel pool -- in the complex's Unit 3 -- is believed to be dangerously low, exposing the stored fuel rods. Without enough water, the rods may heat further and spew out radiation.

"We see it as an extremely serious accident," Yukiya Amano, the head of the U.N.'s International Atomic Energy Agency, told reporters Friday just 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."

Frantic efforts were made Thursday to douse a number of units with water, and authorities were preparing to repeat many of those efforts.

Friday's smoke came from the complex's Unit 2, and its cause was not known, the nuclear safety agency said. An explosion had hit the building on Tuesday, possibly damaging a crucial cooling chamber that sits below the reactor core.

Last week's 9.0 quake and tsunami in Japan's northeast set off the nuclear problems by knocking out power to cooling systems at the reactors. The unfolding crises have led to power shortages in

Japan, forced auto and other factories to close, sending shockwaves through global manufacturing and trade, and triggered a plunge in Japanese stock prices.

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.

"I feel a sense of dread," said Yukiko Morioka, 63, who has seen business dry up at her lottery ticket booth in Tokyo. "I'm not an expert, so it's difficult to understand what's going on. That makes it scarier."

A senior official with the U.N. nuclear agency said Thursday there had been "no significant worsening" at the nuclear plant but that the situation remained "very serious." Graham Andrew told reporters in Vienna that nuclear fuel rods in two reactors were only about half covered with water, and they were also not completely submerged in a third.

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.

At times, the two 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 said Friday that workers hoped to complete the effort in 10 to 15 hours, 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.
The official death toll from the disasters stood at 6,405 as of Friday morning, with 10,259 missing, the national police agency said.

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.

A utility official said Wednesday that the company has been unable to get information such as water levels and temperatures from any of the spent fuel pools in the four most troubled reactors.

Workers have been dumping seawater when possible to control temperatures at the plant since the quake and tsunami knocked out power to its cooling systems, but they tried even more desperate measures on Units 3 and 4.

On Thursday, military helicopters dumped thousands of gallons of water from huge buckets onto Unit 3, and also used military firefighting trucks normally used to extinguish fires at plane crashes.

Officials announced Friday they would not continue with the helicopter drops -- televised footage appeared to show much of that water blowing away -- but would continue spraying from the trucks.

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.

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.

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.