Officials said they hoped to fix the cable to two reactors on Friday and to two others by Sunday, but said work would 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.
U.S. President Barack Obama said the crisis posed no risk to any U.S. territory. He nevertheless ordered a comprehensive review of domestic nuclear plants.