Japanese engineers fighting to cool overheating reactors laid a power line into a stricken nuclear power plant on Saturday as hundreds of thousands of quake-tsunami survivors endured desperate conditions in the frozen north.

Local residents walk amongst the piled up debris from destroyed houses in the city of Rikuzentakata in Iwate prefecture.

In an updated toll, national police said at least 18,000 were dead or missing in Japan\'s worst natural disaster in 88 years. Just under 7,200 were confirmed killed, lost to the tsunami or interred in the wreckage of buildings.

Amid the sea of carnage on Japan\'s northeast coast, one tiny drop of good news seemed to have emerged with the military announcing the rescue of a young man who it said had survived after eight days trapped in his mangled house.

But a spokesman for the Self-Defense Forces later clarified that the man in his 20s was in fact a disaster evacuee who had returned to his house.

Half a million homeless people are struggling to stay warm in freezing temperatures and with scant supplies of food and fuel, after the tsunami reduced whole towns and villages to splintered matchwood.

Further south at the crippled Fukushima No. 1 nuclear plant, crews were locked in what the UN\'s atomic watchdog said was a "race against time" to cool overheating reactors and prevent radiation spewing into the atmosphere.

After an epic week-long tussle to tame the ageing facility, where the tsunami knocked out all-important backup generators, the crews were expecting Saturday to restore electricity to four of its six reactors, officials said.

The nuclear safety agency said workers had got a power line into the plant after the 9.0-magnitude earthquake -- the biggest in Japan\'s recorded history -- felled electricity pylons in the area.

With power back up, the radiation-suited Fukushima engineers hope they can get vital cooling systems online. In the meantime, they have been dumping water by hose and by air on the reactors to avert a feared meltdown.

But given the extent of damage at the plant, it was not yet clear whether the cooling system would work properly even if power is restored.

The lack of power has sent the temperatures of fuel rods -- both in the reactors and in separate containment pools -- soaring as fast-evaporating coolant water leaves them exposed to the air.

The natural disaster on March 11 led to a series of hydrogen explosions and fires at buildings housing the reactor units, stoking anxiety among governments and the public worldwide and contributing to turmoil on financial markets.

But in a televised address Friday evening, Prime Minister Naoto Kan promised the traumatised nation: "We will overcome this tragedy and recover... We will once more rebuild Japan."

Recalling Japan\'s recovery from the ashes of World War II, Kan promised "firm control" of the disaster and said: "We are in a situation in which this crisis is truly testing us as a people."

Japan and its G7 economic allies on Friday intervened jointly in world currency markets for the first time in a decade to calm the turmoil, pushing down the yen as intended and helping to lift battered Tokyo shares.

Japan\'s nuclear agency has hiked the Fukushima accident level to five from four on an international scale measuring up to seven, an admission the crisis now at least equals the 1979 Three Mile Island accident in Pennsylvania.

Japanese and foreign experts are stressing that there is only a very low risk of radiation contamination beyond a 20-kilometre exclusion zone, and say the accident does not compare to the Chernobyl disaster in 1986.

However, fears of radiation hold a terrifying grip in the only country to have suffered a nuclear attack, when US atom bombs in 1945 finally brought Japan to surrender in World War II.

The threat of a nuclear disaster carries a particular resonance for Ayako Ito, who at 84 is old enough to recall the dropping of the US bombs on the cities of Hiroshima and Nagasaki.

"The most difficult part is that you can?t see it but people can just disappear like that," she told AFP at her hillside home in Kamaishi, one of the towns that bore the full force of the towering 10-metre (33-foot) tsunami.

"We\'re already not eating or drinking, and now this is happening to us? It\'s very difficult," she said.

A major international relief operation is under way for the homeless and millions left without water, electricity, fuel or enough food in Japan\'s northeast.

But thick snow has covered the wreckage littering obliterated towns and villages, all but extinguishing hopes of finding anyone else alive in the debris and deepening danger and misery for survivors.

The absence of electricity in the affected areas means little access to television news and newspapers are very hard to come by. So news about the nuclear crisis is often turning into exaggerated and alarming rumour.

Many nations have shifted embassies out of Tokyo, and the mood grew jittery far afield from Japan, with panic-buying of iodine pills in the United States and Asian airports scanning passengers from Japan for radiation contamination.

The vast capital\'s usually teeming streets have been quiet, although some residents headed to work as usual. The city\'s neon glare is dimmed at night, in line with a power-saving drive forced by shutdowns at other atomic plants.

A moment of silence was observed at 2:46 pm on Friday, exactly one week after the earthquake struck.

At one emergency shelter in the town of Yamada, in ravaged Iwate prefecture, hundreds of elderly survivors quietly stood and bowed their heads. Many of them wore face masks and overcoats. Some wiped away tears.

The confirmed dead from the disaster makes it Japan\'s worst natural catastrophe since the 1923 Great Kanto Earthquake, which killed over 142,000 people in the Tokyo region.