A fact-finding mission of the International Atomic Energy Agency (IAEA) said on Wednesday that Japan underestimated the tsunami hazard for its nuclear power plants.

"The tsunami hazard for several sites was underestimated," the U.N. nuclear safety team said in a summary of its report on the nuclear crisis triggered by the twin disasters in March.

It added that nuclear plant designers and operators should appropriately evaluate and protect against the risks of all natural hazards.

The team plans to present the full report of the week-long investigation to an international meeting in Vienna on June 20-24.

The group, consisting of experts from 12 nations, visited three Japanese nuclear power facilities, including the Fukushima No.1 nuclear power plant which was crippled by the March 11 earthquake and tsunami.