WASHINGTON, (AFP) - The risk of thyroid cancer among people who were exposed as children to the nuclear fallout at Chernobyl has not declined nearly 25 years after the disaster, said a study released Thursday in the United States.

The National Institutes of Health-led study examined more than 12,500 people who were under 18 at the time of the Chernobyl accident on April 26, 1986, and who lived near the accident site in one of three parts of Ukraine.

Each person's thyroid radioactivity levels were measured within two months of the accident, and they were screened for thyroid cancer four times, beginning as early as 12 years after the disaster and continuing for 10 years.

Sixty-five of those in the study were diagnosed with thyroid cancer.

When researchers examined the cancer risk in relation to how much exposure to radioactive iodine-131 (I-131) each person received, they found a two-fold increase for each additional gray, an international unit of absorbed radiation.

"The researchers found no evidence, during the study time period, to indicate that the increased cancer risk to those who lived in the area at the time of the accident is decreasing over time," said the study.

Overall, the "clear dose-response relationship, in which higher absorption of radiation from I-131 led to an increased risk for thyroid cancer... has not seemed to diminish over time," it said.

The study was carried out by an international team of scientists and was headed by the National Cancer Institute, which is part of the NIH. It appears
in the March 17 issue of the journal Environmental Health Perspectives.

Separate studies involving people who survived atomic bombs have shown that cancer risk begins to decline after 30 years, but remains high after 40 years compared to the general population.

The study authors said more follow up is needed to judge if or when such a decline may be occurring among survivors of Chernobyl, the worst nuclear
accident ever.

The disaster, whose memory has been revived with Japan's attempts to contain overheating at its Fukushima nuclear plant after a 9.0-magnitude
earthquake and massive tsunami, involved a full-scale nuclear explosion in the
number-four reactor at Chernobyl as a result of human error during a botched
testing procedure.

Some five million people are believed to have been affected by the disaster
in Belarus, Russia and Ukraine, where millions of acres (hectares) of agricultural and forest land remain contaminated.

The disaster's death toll is hotly debated. UN agencies estimate up to 9,000 people could be expected to die as a direct consequence of the accident,
and that the disaster will end up costing hundreds of billions of dollars.

Environmental groups such as Greenpeace say up to 100,000 people could die.