A rare photo showing the mushroom cloud from the Hiroshima atomic bombing in two distinct parts, one above the other, has been discovered in the city, a museum curator said Wednesday.

The black-and-white picture is believed to have been taken about half-an-hour after the bombing on August 6, 1945, around 10 kilometres (six miles) east of the hypocentre.

"The existence of this shot was always known in history books, but this is the first time that the actual print has been discovered," said a curator at the Hiroshima Peace Memorial Museum.

"A shot showing the mushroom cloud split into two like this is very rare."

The photo was found among articles related to the atomic bombing now owned by Honkawa Elementary School in Hiroshima city, she said.

The best-known pictures of the aftermath of the bombing were taken from the air by the US military.

An American B-29 bomber named Enola Gay dropped the atomic bomb nicknamed "Little Boy", turning the western Japanese city into a nuclear inferno and killing an estimated 140,000 in the final chapter of World War II.

Three days later another atomic bomb -- "Fat Man" -- was dropped on the city of Nagasaki, claiming the lives of another 70,000.