Japanese officials expressed concerns about the quality of Tokyo's drinking water Wednesday, as workers continued to face an uphill battle containing leaks at a local nuclear plant.

Authorities said the level of radioactive iodine in some city tap water is two times the recommended limit. Officials cautioned against giving the water to young children, but said there was no immediate health risk for adults.

"It is really scary. It is like a vicious negative spiral from the nuclear disaster," Etsuko Nomura, a mother of two young children ages two and five told the Associated Press. "We have contaminated milk and vegetables, and now tap water in Tokyo, and I'm wondering what's next."

In a precautionary move, the U.S. Food and Drug Administration halted imports of dairy and produce from the area of Japan affected by the radiation leak.

"The FDA, together with other agencies, is carefully monitoring any possibility for distribution of radiation to the United States," read a statement posted on the FDA's website late Tuesday.

"At this time, theoretical models do not indicate that significant amounts of radiation will reach the U.S. coast or affect U.S. fishing waters," the statement read.

Meanwhile, workers were evacuated from Japan's Fukushima Dai-ichi power plant Wednesday after a stream of black smoke was detected rising from unit three of the damaged nuclear complex, according to the Tokyo Electric Power Co.

"We don't know the reason" for the smoke, said Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency.

The incident was the latest obstacle in preventing a nuclear meltdown in the wake of the March 11 earthquake and tsunami.