TOKYO—Raising concerns over possible contamination from the stricken Fukushima Daiichi nuclear plant, Japan's health ministry said Saturday that an abnormal amount of a radioactive material was detected in spinach grown about 110 kilometers (about 70 miles) northeast of Tokyo.

The ministry also said that an abnormal amount of the material, called iodine-131, was detected in milk from a dairy farm about 50 kilometers away from the plant.

"This is the first time ever that an abnormal amount of radioactive material has been detected in food in Japan," said ministry official Ryusuke Hagiwara.

Chief Cabinet Secretary Yukio Edano said that the amounts detected in the milk and spinach would not immediately affect human health. Even so, concerns over possible food contamination mark a serious setback in Japan's attempts to contain its worst-ever nuclear accident as it threatens to widen the economic impact of the disaster to Japan's declining farming industry.

Also Saturday, Japan's science ministry said that small amounts of a radioactive substance not usually present have been detected in tap water in Tokyo and five nearby prefectures, but it said the amounts are too small to pose a threat to human health. The ministry started monitoring tap water for radioactive material Saturday, and will continue to check it every day.