Tokyo: The UN atomic watchdog International Atomic Energy Agency (IAEA) said on Wednesday the Japan underestimated the hazard posed by tsunamis to nuclear plants.

"The tsunami hazard for several sites was underestimated," said the IAEA team in the preliminary report it handed to Japan's centre-left government, ahead of a full report to be presented in Vienna later this month.

Praising Tokyo's response to the March 11 disaster as "exemplary", the IAEA also stressed the importance of "regulatory independence and clarity of roles", touching on the fact that Japan's nuclear watchdog is part of the ministry of trade and industry, which promotes atomic power.

The twin-disasters had rocked the country on March 11 causing the world's worst nuclear disaster since Chernobyl 25 years ago at the Fukushima Daiichi plant, which has since leaked radiation into the air, ground and sea.

The IAEA sent an 18-member team of its own experts and specialists from 12 countries, including the United States, China, Russia and South Korea, on a fact-finding mission to Japan.

"Nuclear designers and operators should appropriately evaluate and protect against the risks of all natural hazards, and should periodically update those assessments and assessment methodologies," it said.

The 14-metre (45-foot) wave that slammed into the plant knocked out reactor cooling systems and backup power generators, causing partial reactor meltdowns and forcing emergency crews to douse reactors with water since then.