The amounts do not exceed government safety limits, but tests on water, which for decades were only done once a year, usually show no iodine.
Japan's Health Ministry also said radioactive iodine was found in drinking water in Fukushima prefecture, slightly exceeding government safety limits on Thursday. On Friday, levels were about half that benchmark, and by Saturday they had fallen further.
These findings are adding to the public's concerns about radiation leaking from the crippled nuclear plant in the island nation's northeast coast.
High levels of radiation have already been found in some batches of milk and spinach in farms near the nuclear plant. While the radiation levels exceed the limits allowed by the government, Chief Cabinet Secretary Yukio Edano insisted the products "pose no immediate health risk."
The tainted milk was found 20 miles (30 kilometers) from the plant, a local official said. The spinach was collected from six farms between 60 miles (100 kilometers) and 75 miles (120 kilometers) to the south of the reactors.
Iodine levels in the spinach exceeded safety limits by three to seven times, a food safety official said. Tests on the milk done Wednesday detected small amounts of iodine 131 and cesium 137, the latter being a longer lasting element and can cause more types of cancer. But only iodine was detected Thursday and Friday, a Health Ministry official said.
Officials tried to calm public jitters, saying the amounts detected were so small that people would have to consume unimaginable amounts to endanger their health.
Edano said someone drinking the tainted milk for one year would consume as much radiation as in a CT scan; for the spinach, it would be one-fifth of a CT scan. A CT scan is a compressed series of X-rays used for medical tests.
Officials in Japan's 47 prefectures have been asked to test agricultural products, seafood and drinking water for possible contamination.
Many people in Japan have stopped buying fish and other seafood and restaurants and hotels around the world are also following suit.
The Food and Drug Administration said it will monitor foods imported from Japan to the United States.
Meanwhile, emergency crews at the Fukushima Dai-ichi plant continued working to restore power and get the cooling systems back up and running. They hope to have electricity back on sometime Sunday. Edano said conditions at Units 1, 2 and 3 - rocked by explosions in the past eight days - had "stabilized."
A fire truck with a high-pressure cannon was parked outside the plant's Unit 3, about 300 meters (yards) from the Pacific coast, and began shooting a stream of water nonstop into the pool for seven straight hours, said Kenji Kawasaki, a spokesman for the nuclear safety agency.
A separate pumping vehicle will keep the fire truck's water tank refilled. 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.
Overheated nuclear materials have been leaking radiation since last week's magnitude-9.0 earthquake and following tsunami, and experts say the problem won't be fixed any time soon.
"Whatever the resolution is, it's going to take time. It's going to take weeks and months, not days. It's still too much of an active site, too much water, too much unknown conditions," said Gary Was of University of Michigan.
The Japanese government admitted it was slow to respond to the nuclear troubles, which added another crisis on top of natural disasters, which officials believe killed more than 10,000 people and displaced more than 400,000 others.
Evacuations were ordered within a 12-mile radius of the damaged nuclear plant. Evacuation centers are crowded and many who are still in their homes are left without electricity, phone service and food.
The casualty toll from the devastating earthquake and tsunami has risen to 8,133 with more than 12,272 people still officially missing. In cities that do have electricity, rolling blackouts are expected to last for months.
The Associated Press contributed to this report.