Japanese authorities have told the U.N.'s atomic watchdog they are making preparations to distribute iodine to people living near nuclear power plants affected by Friday's earthquake, the Vienna-based agency said.
Iodine can be used to help protect against thyroid cancer in the case of radioactive exposure in a nuclear accident.
After the 1986 Chernobyl disaster, thousands of cases of thyroid cancer were reported in children and adolescents who were exposed at the time of the accident. More cases are expected.
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 U.N. 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.