Helicopter, Japan - AP - Mar. 17, 2011
A helicopter takes off from a Japan Ground Self Defense Force base in Sendai, Japan, Thursday, March 17, 2011. Photo by AP
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Japanese engineers raced on Friday to restore a power cable to a crippled nuclear power plant in the hope of restarting pumps desperately needed to pour cold water on overheating fuel rods and avert a catastrophic release of radiation.

Officials could not forecast when the cable might be connected, but said work would stop on Friday morning to allow helicopters and fire trucks to resume pouring water on the Daiichi plant, about 240 km north of Tokyo.

"Preparatory work has so far not progressed as fast as we had hoped," an official of plant operator Tokyo Electric Power Co (TEPCO) told a news briefing, adding that a cold snap was hampering the effort, as was the need to constantly check radiation levels were safe for the engineers to work.

Washington and other foreign capitals have expressed growing alarm about radiation leaking from the plant, severely damaged by a magnitude 9.0 earthquake and tsunami a week ago that triggered a series of destructive explosions which compromised the nuclear reactors and spent fuel storage tanks.

Worst case scenarios would involve millions of people in Japan threatened by exposure to radioactive material, but prevailing winds are likely to carry any contaminated smoke or steam away from the densely populated Tokyo area to dissipate over the Pacific ocean.

U.S. President Barack Obama, however, said the Japan crisis posed no risk to any U.S. territory -- although he nevertheless ordered a comprehensive review of domestic nuclear plants.

"We do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska, or U.S. territories in the Pacific," Obama said. "That is the judgment of our Nuclear Regulatory Commission and many other experts."

Yukiya Amano, head of the Vienna-based International Atomic Energy Agency (IAEA), was due back in his homeland later on Friday with an international team of experts after earlier complaining about a lack of information from Japanese authorities on the crisis.

Graham Andrew, his senior aide, said the situation at the plant was serious but "reasonably stable".

"It hasn't got worse, which is positive," he said. "The situation remains very serious but there has been no significant worsening since yesterday."

Even if TEPCO manages to connect the power, it is not clear the pumps will work as they may have been damaged by the natural disaster or subsequent explosions. Work has been slowed by the need to frequently monitor radiation levels to protect workers.

U.S. officials took pains not to criticize Japan's government, but Washington's actions indicated a divide with its close ally about the perilousness of the world's worst nuclear accident since the Chernobyl disaster in 1986.

The top U.S. nuclear regulator said the cooling pool for spent fuel rods at the complex's reactor No.4 may have run dry and another was leaking.

Gregory Jaczko, head of the U.S. Nuclear Regulatory Commission, told a congressional hearing that radiation levels around the cooling pool were extremely high, posing deadly risks for workers still toiling in the wreckage of the power plant.

Japan's nuclear agency said it could not confirm if water was covering the fuel rods. The plant operator said it believed the reactor spent-fuel pool still had water as of Wednesday, and made clear its priority was the spent-fuel pool at the No.3 reactor.

On Thursday, military helicopters dumped about 30 tonnes of water, all aimed at this reactor. One emergency crew temporarily put off spraying the same reactor with a water cannon due to high radiation, broadcaster NHK said, but another crew later began hosing it.

Latest images from the plant showed severe damage, with two of the buildings a twisted mangle of steel and concrete.

"The worst-case scenario doesn't bear mentioning and the best-case scenario keeps getting worse," Perpetual Investments said in a note on the crisis.