Emergency workers racing to cool dangerously overheated uranium fuel were today scrambling to connect Japan's crippled reactors to a new power line.
Electricians were fighting tsunami-shattered equipment to restart the complex's cooling systems.
The power line reached the complex yesterday, making the final link without setting off a spark -- and potentially an explosion at the Fukushima Dai-ichi plant on Japan's north-east coast.
"Most of the motors and switchboards were submerged by the tsunami and they cannot be used," said Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency.
Operators of the plant, which has prompted global worries of radiation leaks, hope to have power reconnected to four of the complex's six units today and another tomorrow.
However, even once the power is reconnected, it is not clear if the cooling systems will still work.
Meanwhile, as Japan crossed the one-week mark since the twin natural disasters spawned the nuclear crisis, the government conceded it was slow to respond and welcomed help from the US in hopes of preventing a complete meltdown at Fukushima.
Emergency crews at the nuclear plant faced two continuing challenges: cooling the nuclear fuel in reactors where energy is generated; and cooling the adjacent pools where thousands of used nuclear fuel rods are stored in water.
"In hindsight, we could have moved a little quicker in assessing the situation and co-ordinating all that information and provided it faster," chief cabinet secretary Yukio Edano said.
Japan's government raised the accident classification for the nuclear crisis from Level 4 to Level 5 on a seven-level international scale yesterday.
That put it on a par with the Three Mile Island accident in Pennsylvania in 1979.
Mr Edano also said Tokyo was asking Washington for additional help, a change from a few days ago.
A US military fire truck was among a fleet of Japanese vehicles that sprayed water into Unit 3, sending tonnes of water arcing over the facility in an attempt to prevent nuclear fuel from overheating and emitting dangerous levels of radiation.
Additionally, the US conducted over-flights of the reactor site, strapping sophisticated pods on to aircraft to measure radiation aloft.
While nuclear experts have been saying for days that Japan was underplaying the severity of the crisis, Mr Nishiyama of the nuclear safety agency said the rating was raised when officials realised that at least 3pc of the fuel in three of the complex's reactors had been severely damaged.
That suggested those reactor cores have partially melted down and thrown radioactivity into the environment.
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 complex itself.