US authorities sought Wednesday to reassure Americans that there is minimal health risk here of radioactivity from Japan, as a US iodide pill maker reported an "enormous" run on the drug.

Demand for potassium iodide, which can protect against the effects of radioactive iodine, was strongest on the US West Coast, where some fear a cloud spewing from Japan's Fukushima nuclear plant could be blown, drug company Anbex said.

The firm, which says it is the only US maker of the pills, was flooded with thousands of orders for its Iosat drug after last Friday's earthquake and tsunami, which has triggered an ongoing nuclear crisis.

"The spike is enormous ... we were out of stock by Friday night," said Alan Morris, president of Anbex, which supplies the drug to individuals and retailers, including online.

"The demand mostly is coming from the West Coast of the US, but there are a significant number of inquiries, requests, orders coming from Japan, Korea, all over the Far East," he told AFP.

A random survey of Los Angeles pharmacies by an AFP photographer found no lines of people trying to buy the drug, although some retailers said they had received some requests, but did not have supplies.

The surge in demand came as the head of the US Nuclear Regulatory Commission warned of "extremely high" radiation levels from the Fukushima plant.

US authorities have repeatedly said there is minimal risk of radioactivity reaching the US mainland, while meteorologists say it is difficult to predict exactly how far a radioactive cloud would spread across the Pacific.

The California Department of Public Health's interim director, Howard Backer, also stressed the risks involved in taking potassium iodide unnecessarily.

"We urge Californians to not take potassium iodide as a precautionary measure," he said.

"It is not necessary given the current circumstances in Japan, it can present a danger to people with allergies to iodine, shellfish or who have thyroid problems, and taken inappropriately it can have serious side effects," Backer added.

In one apparent miscommunication, US Surgeon General Regina Benjamin appeared to contradict the reassuring message during a visit to San Francisco on Tuesday.

"We can't be overprepared -- we learned that with 9/11, we learned that with Katrina and we learned that this week with the tsunami," she told an NBC reporter. "Even if it's one life we save by being prepared, it's worth it."

A spokeswoman clarified her position on Wednesday, saying Benjamin had not heard about panicked California residents stocking up on potassium iodide.

"She commented that it is always important to be prepared. However she wouldn't recommend that anyone go out and purchase (the drug) for themselves at this time," said spokeswoman Kate Migliaccio, according to the Los Angeles Times.

Anbex chief Morris said his drug company, which developed the product after the Three Mile Island nuclear disaster in 1979, hoped to have new stocks of potassium iodide pills ready to ship in two weeks.

His company was the only US manufacturer of potassium in pill form, he said, adding that there was a liquid form available from a company called Fleming Pharmaceuticals.

A statement on the Fleming's website said the firm was "running nearly around the clock as employees ship potassium iodide to Japan."

Radioactive iodine from a nuclear event can pollute the air and contaminate the food supply. Experts believe many cancer cases after the Chernobyl disaster in Ukraine in 1986 were linked to milk from contaminated cows.

Thyroid glands quickly absorb radioactive iodine, causing damage. But iodide pills can block radioactive iodine from being taken into the thyroid gland, according to a fact sheet by the US Centers for Disease Control.