Iodine can be used to help protect the body from radioactive exposure.

In Japan on Saturday, radiation leaked from a damaged nuclear reactor after an explosion blew the roof off in the wake of the massive earthquake, but the government insisted that radiation levels were low.

Japan's Jiji news agency later said three workers suffered radiation exposure near the Fukushima nuclear plant.

The International Atomic Energy Agency (IAEA), the UN nuclear body, said Japanese authorities had informed it of the explosion and that they were "assessing the condition of the reactor core".

Japan expanded the evacuation zone around the plant, Fukushima Daiichi, and also that of the nearby Fukushima Daini nuclear power plant.

"The authorities also say they are making preparations to distribute iodine to residents in the area of both the plants," the IAEA said in a statement.

"The IAEA has reiterated its offer of technical assistance to Japan, should the government request this," it said.