"71 years ago, death fell from the sky and the world was changed," he said after laying a large floral wreath.

Obama, whose predecessor Harry Truman made the decision to launch the world's first nuclear strike, greeted ageing survivors and embraced one elderly man who appeared overcome with emotion.

He also chatted with a smiling Sunao Tsuboi, 91, who had earlier said he wanted to tell the US president how grateful he was for his visit.

Obama, wearing a dark suit, looked sombre as he offered a wreath at the cenotaph, in the shadow of a wrecked building that stands in silent tribute to the dead.

The president lowered his head and closed his eyes as he paused for a moment's contemplation, before withdrawing and watching Japanese Prime Minister Shinzo Abe do the same.

"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," Obama said to the assembled crowd.

"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."

"The world was forever changed here but, today, the children of this city will go through their day in peace," the US president said. "What a precious thing that is."

(AFP)