Japan has reported some progress in stabilising one of six tsunami-crippled nuclear reactors at the Fukushima Daiichi power plant.
Engineers are racing to restore power to the stricken power plant to cool it and prevent a greater catastrophe.
Fire trucks sprayed water for about three hours on reactor 3, widely considered the most dangerous at the ravaged nuclear complex because of its use of highly toxic plutonium.
"The situation there is stabilising somewhat," chief cabinet secretary Yukio Edano told a news conference.
Engineers earlier attached a power cable to the outside of the mangled plant in a desperate attempt to get water pumps going that would cool overheating fuel rods and prevent a deadly radiation leak.
They hope electricity will flow by tomorrow to four reactors in the complex, which lies about 240km north of Tokyo.
Japan also reported its first contamination of food since last week's earthquake and tsunami that left nearly 18,000 people dead or missing, turned entire towns into debris-strewn wastelands and triggered a nuclear emergency.
Japan ordered a halt to all food product sales from Fukushima prefecture, said the International Atomic Energy Agency (IAEA) which warned that radioactive iodine found in food can pose a short-term risk to human health.
Radiation levels in milk from a Fukushima farm about 30km from the plant, and spinach grown in Ibaraki, a neighbouring prefecture, exceeded government limits, Edano said.
Faint radiation was also found in tap water in Maebashi, 100km north of Tokyo, Kyodo News reported.
Mr Edano told reporters before the IAEA warning that higher radiation levels posed no risk to human health, but the findings are sure to heighten scrutiny of Japanese food exports, especially in Asia, their biggest market.
Prime minister Naoto Kan, facing Japan's biggest disaster since the second World War, sounded out the opposition about forming a government of national unity to deal with this crisis.
Officials connected a power cable to reactor 2 today and planned to test power in reactors 1, 2, 3 and 4 tomorrow.
Working inside a 20km evacuation zone at Fukushima, nearly 300 engineers got a second diesel generator attached to reactor 6 working, the nuclear safety agency said. They used the power to restart cooling pumps on No 5.
Nearly 1.5km of cable is being laid before engineers try to crank up the coolers at reactor 2, followed by numbers 1, 3 and 4 this weekend, company officials said.
"If they are successful in getting the cooling infrastructure up and running, that will be a significant step forward in establishing stability," said Eric Moore, a nuclear power expert at US-based FocalPoint Consulting Group.
If that fails, one option is to bury the sprawling 40-year-old plant in sand and concrete to prevent a catastrophic radiation release. The method was used at the Chernobyl reactor in 1986, scene of the world's worst nuclear reactor disaster.
Japan has raised the severity rating of the nuclear crisis to level 5 from 4 on the seven-level INES international scale, putting it on a par with the Three Mile Island accident in Pennsylvania in 1979. Chernobyl, in Ukraine, was a 7 on that scale.
The operation to avert large-scale radiation has overshadowed the humanitarian crisis caused by the 9.0-magnitude quake and tsunami.
Some 390,000 people, many elderly, are homeless, living in shelters in near-freezing temperatures in northeastern coastal areas. Food, water, medicine and heating fuel are in short supply. Nearly 290,000 households in the north still have no electricity and about 940,000 lack running water.
Aid groups say most victims are getting help, but there are pockets of acute suffering. “We've seen children suffering with the cold, and lacking really basic items like food and clean water," Stephen McDonald of Save the Children said in a statement.