An increase in thyroid cancer among children is unlikely after the meltdown at Japan's Fukushima nuclear plant four years ago, but it remains unclear exactly how much radiation children in the vicinity of the plant were exposed to, the United Nations' nuclear watchdog said in a report released on Monday.

Increased thyroid cancer is generally the leading health concern after exposure to nuclear radiation, but that may not be the case after the accident at Tokyo Electric Power Co's Fukushima Daiichi plant in March 2011, the Vienna-based International Atomic Energy Agency said.

"Because the reported thyroid doses attributable to the

accident were generally low, an increase in childhood thyroid cancer attributable to the accident is unlikely," the report said.

"However, uncertainties remained concerning the thyroid equivalent doses incurred by children immediately after the accident," it added.

Those uncertainties are largely due to a lack of reliable personal radiation monitoring data immediately after the accident, which released radioiodine and other radioactive materials into the environment, the report said.

In the case of Fukushima, the earthquake and following tsunami cut off power to the plant and made emergency response measures difficult, if not impossible, to implement.

Adding to the uncertainty was the fact that the administration of "stable iodine" to protect children's thyroid glands was not done uniformly at the time, "primarily due to the lack of detailed arrangements," the report said.

Detailed screening of children's thyroid glands is being undertaken now in Japan as part of a survey aimed at the early detection and treatment of diseases.

The report highlighted areas where improvements were needed in the wake of the Fukushima accident. The IAEA said more sustainable solutions were needed for the management of highly radioactive water and radioactive waste being collected at the plant, "including the possible resumption of controlled discharge into the sea."

It added that countries should prepare detailed scenarios and train workers for coping with worst-case natural disasters, including situations where more than one disaster is combined with a nuclear accident. They should also plan for clean-up operations in the wake of such incidents.

The report also called for strengthened international cooperation in the event of such accidents.

Japan began encouraging residents to return to homes 12 miles (19 km) from the plant last year, but many residents have mixed feelings about returning to abandoned towns.