Radiation from a tsunami-battered nuclear power plant in northeast Japan is causing growing concerns about the country's food and water safety — and sparking import restrictions around the globe.

Water at Tokyo's main water purification plant has tested above the legal limits for radioactive iodine, prompting a new alert Wednesday.

People were warned not to give tap water to babies one year or younger. The announcement caused a surge in the sales of bottled water and other packaged beverages.

Levels of radioactive iodine at the treatment plant, which draws water from local rivers, contained 210 becquerels per litre of iodine-131, officials from the Tokyo Water Bureau said at a news conference. They said the level is more than twice the amount considered safe for infants to consume, adding the level is not an immediate health risk for adults.

Dairy farmer Kenichi Hasegawa dumps milk in a corn field in Iitatemura in Fukushima prefecture on Wednesday. (Takuya Yoshino/Yomiuri Shimbun/AP)

"These limits have been set taking into consideration consumption of water over a long period. If there are no alternatives, it can be consumed," said Ei Yoshida, manager of the Tokyo Metropolitan Government water purification section.

Nearly two weeks after the twin March 11 disasters, nuclear officials are still struggling to stabilize the damaged and overheated Fukushima plant, which has been leaking radiation since the disasters knocked out its cooling systems.

Nuclear workers have had to contend with a number of setbacks, with explosions and fires in four of the facility's six reactors causing delays.

A plume of black smoke forced workers inside Unit 3 to evacuate Wednesday, though officials said Thursday the smoke had stopped.

In the meantime, white smoke rose intermittently from two of the other units. The Japanese Broadcasting Corporation reported steam rising from four of the six units early Thursday.

As a precaution, officials have evacuated residents within 20 kilometres of the plant and advised those up to 30 kilometres away to stay indoors to minimize exposure.

Radiation hits food, water

Radiation has seeped into vegetables, raw milk, the water supply and even seawater in the areas surrounding the plant. Broccoli was added early Wednesday to a list of tainted vegetables that already includes spinach, canola and chrysanthemum greens.

Other potentially contaminated vegetables include:

• Cabbage
• Parsley
• Leeks
• Radishes
• Cucumbers
• Cauliflower
• Leafy vegetables, including kukitachina and kakina.

Residents of cities in Japan's northeast earlier had been advised not to drink tap water due to elevated levels of radioactive iodine, which can cause thyroid cancer. Until Wednesday, levels found in Tokyo tap water had been minute, according to officials.

The U.S. Food and Drug Administration announced Tuesday that it will halt imports of dairy products and produce from the Fukushima region. Other foods imported from Japan, including seafood, will still be sold to the public but screened first for radiation, the agency said.

Japanese foods make up less than four per cent of all U.S. imports, and the FDA has said it expects no risk to the U.S. food supply from radiation.

Canada restricts food imports

Canada is tightening restrictions on food imported from Japan, demanding documents proving that products coming from areas of Japan affected by the nuclear crisis — including milk, fruit and vegetables — are safe.

The Canadian Food Inspection Agency (CFIA) on Wednesday said it's working with Canada Border Services Agency to enforce its decision.

"These products [from the Japanese prefectures of Fukushima, Gunma, Ibaraki, and Tochigi] will not be allowed entry into Canada without acceptable documentation verifying their safety," CFIA said.

Earlier, the agency said the events unfolding in Japan "pose no current risk to food in Canada."

"Rigorous controls and tracking systems are in place for imported food, plants and livestock, and additional actions will be taken as necessary to protect the integrity of Canada's food supply," CFIA said.

The total volume of agri-food imports from Japan to Canada was approximately $42.6 million in 2010, which was about 0.3 per cent of the food entering Canada, according to the CFIA.

Japanese products currently available for sale in Canada were shipped prior to the earthquake and tsunami, CFIA said.

Hong Kong is blocking food imports from five Japanese prefectures over concerns about radiation. Its ban covers Fukushima, Ibaraki, Tochigi, Gunma and Chiba prefectures and exempts some foods that have been certified as safe.

The ban takes effect Thursday and applies to milk, meat, vegetables and seafood harvested, processed or packed since March 11, when the quake and tsunami struck.

Australia does not import fresh food from Japan, but its food safety regulator has announced a halt to Japanese imports such as sauces and seaweed from four prefectures near the crippled nuclear plant as a precaution.

Officials in Germany have started extra checks on Japanese food imports to ensure they are free from radioactivity, Reuters reported.

There are also reports that tiny amounts of radiation from the nuclear complex in Fukushima have been detected in the atmosphere over Iceland, but does not pose health risks.

With files from The Associated Press