"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 over 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.

There are claims that the effects of nuclear exposure have been observed on successive generations of descendants of people living in Hiroshima and Nagasaki when the US dropped atomic bombs in the final days of World War II.

But Otaki warned 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.

Kunikazu Noguchi, associate professor in radiological protection at Nihon University School of Dentistry, also said more data was needed to determine the impact of the Fukushima accident on animals in general.

"This is just one study," Noguchi said. "We need more studies to verify the entire picture of the impact on animals."

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