TOKYO—Japanese authorities said Wednesday they found radioactivity in Tokyo tap water at a level potentially harmful long-term to infants, as the country's nuclear crisis broadened into a major public health issue.

Food fears grow in Japan after the government released a longer list of vegetables from the area around Fukushima showing elevated levels of radioactivity. WSJ's Mariko Sanchanta and Yumiko Ono discuss.

Officials cautioned that the levels of radioactive iodine found in Tokyo's tap water is still safe for adults, while contamination found in food and water elsewhere in the country remains below levels that could cause immediate health damage.

Still, Wednesday's findings suggest a new front is unfolding in Japan's fight to contain radioactive material at the heavily damaged Fukushima Daiichi nuclear power complex in northeast Japan.

Radiation levels in the air in Tokyo were at four times normal levels Wednesday, though still within government limits. Officials said the heightened levels could have been due to recent rainstorms that brought radioactive material down from the air—the same possible cause to which they attributed the heightened radiation levels in one of three Tokyo water facilities tested this week.