Edano said, however, that conditions at the plant's most dangerous No. 3 reactor unit have likely become relatively stable after firefighters threw some 60 tons of water at a boiling spent fuel pool there shortly after midnight from outside the damaged building housing it.
Defense Minister Toshimi Kitazawa separately said the surface temperatures at the No. 1 to No. 4 reactors were found in the morning at 100 C or lower by a Self-Defense Force helicopter, adding their conditions remain more stable than expected.
''We're trying to get things under control, but we're still in an unpredictable situation,'' Edano said.
Prime Minister Naoto Kan instructed the Defense Ministry to keep monitoring around the plant, Kitazawa said.
Plant operator Tokyo Electric Power Co., meanwhile, managed to connect power cables to the No. 1 and No. 2 reactor buildings, paving the way for checks of their equipment as early as Sunday to see if they can work.
Restoring a stable source of electricity to reactivate lost cooling systems is a key step to prevent further deterioration of the situation, particularly as the No. 2 reactor has suffered a rupture at its containment vessel's pressure-suppression chamber at the bottom.
At the No. 3 reactor, the Tokyo Fire Department's ''hyper rescue'' team began in the afternoon an unmanned operation of having its heavy fire trucks continue shooting a total of 1,260 tons of water at its spent fuel pool for seven hours, the department said.
After massive smoke was detected from the No. 3 reactor building on Wednesday, the Self-Defense Forces, firefighters and others have been engaged in an unprecedented mission to spray tons of water in an attempt to fill its pool with water, which is vital to prevent a release of radioactive substances.
A rise in water temperature, usually to 40 C, causes the water level to fall, thus exposing the spent nuclear fuel rods, which could then heat up further, melt and discharge highly radioactive materials in a worst-case scenario, experts say.
The SDF is now preparing to spray water into the No. 4 reactor to help cool its spent fuel pool as well, Edano said.
At the No. 5 and No. 6 reactors, it has become possible to cool spent fuel by circulating water in the storage pools, as one of emergency generators was restored early Saturday, leading the temperature to fall to 67.6 C from 68.8 C in four hours, said Tokyo Electric, or TEPCO.
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