Officials said they hoped to fix the cable to two reactors on March 18 and to two others by March 20, but said work will stop in the morning to allow helicopters and fire trucks to resume pouring water on the Fukushima Daiichi plant, about 240 km (150 miles) 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 engineers had to be constantly checked for radiation levels.

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 and 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.

Yukiya Amano, head of the Vienna-based International Atomic Energy Agency (IAEA), was due back in his homeland later on March 18 with an international team of experts after earlier complaining about a lack of information from Japan.

Graham Andrew, his senior aide, called the situation at the plant "reasonably stable" but the government said white smoke or steam was still rising from three reactors and helicopters used to dump water on the plant had shown exposure to small amounts of radiation.

Even if TEPCO manages to connect the power, it is not clear the pumps will work as they may have been damaged in the earthquake or subsequent explosions.