Abnormal levels of radiation have been detected in milk and spinach near a nuclear power plant damaged when the northeast of the country was battered by a tsunami
The discovery came as engineers tried to restore power to vital cooling systems at the stricken Fukushima 1 site.
The contaminated milk was found in Fukushima prefecture, where the power station is located, while the tainted spinach was discovered in neighbouring Ibaraki prefecture, government spokesman Yukio Edano said.
He said the health ministry had ordered authorities in both prefectures to check where the products came from, how they were distributed and possibly suspend sales.
However, Mr Edano urged calm, saying that the contaminated milk posed little health risk as drinking it for a year would only expose consumers to radiation the equivalent of one medical CT scan.
"It's not like if you ate it right away you would be harmed," he said.
"It would not be good to continue to eat it for some time."
Four of Fukushima's six reactor units have had fires, explosions or partial meltdowns in the week since the country was hit by a massive earthquake and tsunami.
The US has said it could take weeks to cool the overheated fuel rods.
Low levels of radiation have been detected well beyond Tokyo, which is 140 miles (220km) south of the plant, but hazardous levels have been limited to the plant itself.
But hopes that a nuclear disaster could be avoided were raised on Saturday as engineers reported some success in their battle to cool the uranium fuel rods in the reactors.
Sky News presenter Colin Brazier, in Tokyo, said: "The Japanese nuclear technicians... do finally appear to be entering what looks like, technically, the end game up there.
"Plan A is to make sure that the electric cables that they have successfully attached - certainly to reactor two but also to reactors five and six in the past few hours - work and the nuclear reactor core is cooled down.
"If that doesn't work they'll then resort to Chernobyl-style tactics, pouring on hundreds of tonnes of concrete to entomb all six reactors."
Mr Edano said there had also been some success at reactor three - considered particularly dangerous because it uses highly toxic plutonium - after fire engines sprayed water on it for about three hours.
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.
A spokesman for Tokyo Electric Power Co, which owns and runs the plants, said the generators themselves were not directly exposed to the waves, but some electrical support equipment was outside.
The complex was protected against tsunamis of up to five metres (16 feet), but the tsunami was at least six metres (20 feet).
The crisis has seen Japan's nuclear safety agency raising the rating of its nuclear accident from 4 to 5 on a 7-level International Nuclear Event Scale.
The scale defines a level 4 incident as having local consequences and a level 5 incident as having wider consequences.
Japan's police agency has said nearly have 7,200 people have been confirmed dead and more than 10,900 are still missing following the magnitude 9.0 earthquake and the resulting tsunami.
Prime Minister Naoto Kan has sounded out the opposition about forming a coalition to deal with the crisis, but the leader of the largest opposition party rejected the idea.
Japan, burdened with massive government debt even before the disaster struck, now must handle large-scale humanitarian operations in the ravaged northeast.