Residents cleared store shelves of bottled water after Tokyo Gov. Shintaro Ishihara said levels of radioactive iodine in tap water were more than twice what is considered safe for babies. Officials begged those in the city to buy only what they needed, saying hoarding could hurt the thousands of people without any water in areas devastated by the March 11 earthquake and tsunami.

"I've never seen anything like this," clerk Toru Kikutaka said, surveying the downtown Tokyo supermarket where the entire stock of bottled water sold out almost immediately after the news broke, despite a limit of two, two-liter bottles per customer.

The unsettling new development affecting Japan's largest city, home to around 13 million people, added to growing fears over the nation's food supply.

Radiation from the Fukushima Dai-ichi nuclear plant has seeped into raw milk, seawater and 11 kinds of vegetables, including broccoli, cauliflower and turnips, from areas around the plant. The U.S. Food and Drug Administration said it was halting imports of Japanese dairy and produce from the region near the facility. Hong Kong went further and required that Japan perform safety checks on meat, eggs and seafood before accepting those products.

Officials are still struggling to stabilize the nuclear plant, which on Wednesday belched black smoke from Unit 3 and forced the evacuation of workers, further delaying attempts to make needed repairs. The plant, 140 miles (220 kilometers) north of Tokyo, has been leaking radiation since the quake and tsunami knocked out its crucial cooling systems.

The crisis is emerging as the world's most expensive natural disaster on record, likely to cost up to $309 billion, according to a new government estimate. Police say an estimated 18,000 people were killed.