Genetic mutations have been found in three generations of butterflies from near Japan's crippled Fukushima nuclear plant, scientists say.
Around 12 per cent of pale grass blue butterflies that were exposed to nuclear fallout as larvae immediately after the tsunami-sparked disaster had abnormalities, including smaller wings and damaged eyes, researchers said on Tuesday.
The insects were bred in a laboratory outside the fallout zone and 18 per cent of their offspring displayed similar problems, said Joji Otaki, associate professor at Ryukyu University in Okinawa, southwestern Japan.
The figure rose to 34 per cent in the third generation of butterflies, he said, even though one parent from each coupling was from an unaffected population.
The researchers collected another 240 butterflies in Fukushima in September last year, six months after the disaster. Abnormalities were recorded in 52 per cent, which was "a dominantly high ratio", Otaki told AFP.
The results of the study were published in Scientific Reports, an online research journal from the publishers of Nature.
Otaki later carried out a comparison test in Okinawa exposing unaffected butterflies to low levels of radiation, with the results showing similar rates of abnormality, he said.
"We have reached the firm conclusion that radiation released from the Fukushima Daiichi plant damaged the genes of the butterflies," Otaki said.
The quake-sparked tsunami of March 2011 knocked out cooling systems at the Fukushima Daiichi nuclear power plant, causing three reactors to go into meltdown in the world's worst atomic disaster for 25 years.
The findings will raise fears about the long-term effects of the leaks on people who were exposed in the days and weeks after the accident, as radiation spread over a large area and forced thousands to evacuate.
Otaki, however, said it was too soon to jump to conclusions, saying his team's results on the Fukushima butterflies could not be directly applied to other species, including humans.
He added he and his colleagues would conduct follow-up studies including similar tests on other animals.