The Japanese government has reported that trace amounts of radioactive iodine have been detected in tap water in Tokyo and five other areas, amid concerns about leaks from the crippled Fukushima nuclear power station.
The ministry says the amounts did not exceed government safety limits, but the announcement has added to safety fears among the Japanese people. Earlier in the day, Japan banned the sale of food products from near Fukushima after finding elevated radiation levels in spinach and milk from the area's farms.
The International Atomic Energy Agency said: "Though radioactive iodine has a short half-life of about eight days and decays naturally within a matter of weeks, there is a short-term risk to human health if radioactive iodine in food is absorbed into the human body."
Tainted milk was found 30km (20 miles) from the plant and contaminated spinach was collected up to 100km (65 miles) to the south.
Chief cabinet secretary Yukio Edano told reporters in Tokyo that the radiation levels exceeded the limits allowed by the government, but the products "pose no immediate health risk" and testing was being done on other foods..
"It's not like if you ate it right away you would be harmed," Edano said. "It would not be good to continue to eat it for some time."
Edano said the amount of radiation detected in the milk was the equivalent to one CT scan – the series of X-rays used for medical tests – if consumed continually for a year.
Low levels of radiation have been detected well beyond Tokyo, which is 220km south of the plant, but hazardous levels have been limited to the plant itself.
The food scare is the latest consequence of the cascade of disasters unleashed by the earthquake on 11 March.
Emergency teams scrambled on Saturday to restore power to the Fukushima plant so it could cool dangerously overheated fuel.
Firefighters pumped tons of water directly from the ocean into the cooling pool for used fuel rods at the plant's unit 3. The rods are at risk of burning up and sending a broad release of radioactive material into the environment.
Just outside the bustling disaster response centre in the city of Fukushima, 60km north-west of the plant, government nuclear specialist Kazuya Konno was able to take only a three-minute break for his first meeting with his wife Junko and their children since the earthquake.
"It's very nervewracking. We really don't know what is going to become of our city," Junko told Associated Press. "Like most other people, we have been staying indoors unless we have to go out."
She brought her husband a small backpack with a change of clothes and snacks. The girls, aged four and six and wearing pink surgical masks decorated with Mickey Mouse, gave their father hugs.
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In his latest ministerial update from Tokyo, Edano said: "The situation at the nuclear complex still remains unpredictable. But at least we are preventing things from deteriorating."
A fire truck with a high-pressure cannon parked outside the plant's unit 3 began shooting a continuous arc of water nonstop into the pool for seven hours. Because of high radiation levels, firefighters will only go to the truck every three hours when it needs to be refueled. They expect to pump about 1,400 tons of water, nearly the capacity of the pool.
Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency said backup power systems at the plant had been improperly protected, leaving them vulnerable to the tsunami that ravaged the north-eastern coast of Japale.
The failure enabled uranium fuel to overheat and was a "main cause" of the crisis, Nishiyama said. "I cannot say whether it was a human error, but we should examine the case closely."
A spokesman for Tokyo Electric, which owns and runs the complex, said it was protected against tsunamis of up to five metres (16ft) but a six-metre wave of water struck Fukushima on 11 March.
Plant operators said they would reconnect four of the plant's six reactor units to a power grid on Saturday. Workers have to methodically work through badly damaged and deeply complex electrical systems to make the final linkups without setting off a spark and potentially an explosion.
"Most of the motors and switchboards were submerged by the tsunami and they cannot be used," Nishiyama said.
Even once the power is reconnected, it is not clear if the cooling systems will still work.
The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.
Meanwhile, some Britons in the country have begun their journey back to the UK to escape from radiation fears, power shortages, business closures and a lack of food in shops.
Buses and planes ferried people to safety on Friday, with 24 British nationals leaving tsunami-flattened Sendai on two coaches heading for Tokyo.
The Foreign Office block-booked seats for Britons wanting to fly home on commercial flights, the first of which was a Cathay Pacific flight to Hong Kong, and said two other flights to Hong Kong would be made available on Saturday.
Those directly affected by last week's devastating earthquake and tsunami can fly free of charge, but people wishing to leave Japan who have not been directly affected will pay about £600 per seat.