Obama pays tribute at Hiroshima nuclear memorial

"71 years ago, death fell from the sky and the world was changed," the president said after laying a wreath, as he became the first sitting US leader to visit the site.

Obama looked sombre as he offered the wreath, lowering his head and pausing for a moment with his eyes closed before withdrawing and watching Japanese Prime Minister Shinzo Abe lay his flowers.

"Why did we come to this place, to Hiroshima? We come to ponder a terrible force unleashed in the not so distant past.

We come to mourn the dead," he said.

"Their souls speak to us, they ask us to look inward, take stock of who we are," he said.

"Technological progress without equivalent progress in human institutions can doom us. The scientific revolution that led to the splitting of the atom requires a moral revolution as well."This is why we come to this place, we stand here, in the middle of this city and force ourselves to imagine the moment the bomb fell.

"We force ourselves to feel the dread of children confused by what they see. We listen to a silent cry.