The overriding message that U.S. President Barack Obama conveyed in his historic speech at Peace Memorial Park in Hiroshima is that nuclear weapons should never again be used, an American expert on Japanese history said.

"He (Obama) said memory must never fade, but not because we must remember the suffering, but because we must prevent it now. That is the 'never again' message," said Carol Gluck, a Columbia University professor of Japanese history.

"The memory of Hiroshima is in our actions now to prevent that from happening again and that is why it is a charge to the people," she said on Friday.