Radiation that leaked from the Fukushima nuclear plant following last year's tsunami caused mutations in some butterflies - including dented eyes and stunted wings - though humans seem relatively unaffected, researchers say.

The mutations are the first evidence that the radiation has caused genetic changes in living organisms. They are likely to add to concerns among ordinary people about potential health risks among humans though there is no evidence of it yet. Scientists say more study is needed to link human health with the Fukushima disaster.

The catastrophic meltdowns in three reactors of Fukushima Dai-ichi power plant after it was damaged by the 9.0-magnitude earthquake and tsunami on March 11, 2011 prompted a public backlash against nuclear power, and forced the government to reassess resource-scarce Japan's entire energy strategy.

But the most visible example of the radiation's effect was claimed by a group of Japanese researchers who found radical physical changes successive generations of a type of butterfly, which they said was caused by radiation exposure. They also said that threat to humans - a much larger and longer-lived species - remains unclear.

"Our findings suggest that the contaminants are causing ecological damage. I do not know its implication to humans," Joji Otaki of the University of the Ryukyus in Okinawa, a member of the team that conducted the research, told The Associated Press in an e-mail.

A separate study, released this week, found very low levels of radioactivity in people who were living near the Fukushima plant when it suffered the meltdowns.

The paper, published in the Journal of the American Medical Association, measured cesium levels in 8,066 adults and 1,432 children and found average doses of less than 1 millisievert, which are considered safe. It was the first such study measuring internal exposures to cesium in a large number of people from the disaster.

The research shows contamination decreased over time, particularly among children, in part because more precautions were taken with their food, water and outdoor activity.

"No case of acute health problems has been reported so far; however, assessments of the long-term effect of radiation requires ongoing monitoring of exposure and the health conditions of the affected communities," the report said.

So far, the actual radiation doses inflicted just after the accident are not exactly known, though exposure is thought to be very small, said David J. Brenner, a radiation physicist at Columbia University, who was not part of the research.

"We do need improved estimates of the radiation dose that people in and near Fukushima prefecture actually received," he told the AP. "Right now our estimates are based on very, very rough calculations."

The research on the butterflies was published in Scientific Reports, an open-access online journal by the Nature publication group, which provides faster publication and peer review by at least one scientist.

It says pale grass blue butterflies, a common species in Japan, collected from several areas near the Fukushima plant showed signs of genetic mutations, such as dented eyes, malformed legs and antennae, and stunted wings.

Other experts said they viewed the research as significant.

"Scientists have long known that radiation can be hazardous to human and animal health. Studies of this sort at Fukushima and Chernobyl provide invaluable information concerning just how hazardous radioactive contaminants could be for human populations living in these areas in the future," Tim Mousseau of the University of South Carolina, told the AP by e-mail.

"Butterflies as a group are important bio-indicators for the effects of environmental stressors like radioactive contaminants," said Mousseau, who also is not part of the Japanese research.

The results show the butterflies were deteriorating both physically and genetically, with the share of those showing abnormalities increasing from 12 percent in the first generation to 18 percent in the second and 34 percent in the third.

To study the genetic changes, the scientists raised the new generations of the butterflies in Okinawa, which has not been affected by the radiation releases, mating each abnormal butterfly with one unaffected by such changes.

The researchers also demonstrated the effects of internal exposure to radiation by feeding leaves from plants from the area near the Fukushima nuclear plant to the butterfly larvae.

"The possible risk of internal exposure from ingestion should be investigated more accurately in the near future," it said.

Although the damage is irreversible, the species could develop resistance to the radiation, Otaki said. "In that case, we will observe adaptive evolution," he said.

The research appears to be "a very thorough study," said Jim T. Smith of Britain's University of Portsmouth, also another outsider. The replication of the mutations under lab conditions further supports the report's findings, he said in a telephone interview.

However, he said he would be "very, very wary of trying to extrapolate those results to humans."

Even in the area near Chernobyl in the Ukraine, the absence of humans who left after the 1986 nuclear accident there has actually benefited local fauna in the long run, said Smith, who has conducted research on radiation's effects on aquatic insects.