The United States President Barack Obama on Friday paid tribute to victims of the world's first nuclear attack during a historic visit to Japanese city Hiroshima, embraced a survivor of the attack, but rejected to apologize on behalf of the U.S.

The U.S. forces dropped nuclear weapons on the Japanese cities of Hiroshima and Nagasaki in August 1945, during the final stage of World War II.

"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 U.S. 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.

The Hiroshima and Nagasaki bombings, which killed at least 129,000 people, are the only time a country's army used nuclear weapons during a war.

The bomb "demonstrated that mankind possessed the means to destroy itself".

"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."

Meanwhile, Obama may have faced the legacy of Hiroshima most directly with his embrace of a man who survived the devastating atomic blast.

He spoke briefly with two survivors in the audience for his remarks Friday at Hiroshima Peace Memorial Park: Sunao Tsuboi, the 91-year-old head of a survivors group, and Shigeaki Mori, 79, a historian who was just 8 when the bomb detonated on Aug. 6, 1945.

Obama spoke to Tsuboi first. They laughed at one point, the president throwing back his head and smiling broadly. Obama mostly listened, though, holding the elderly man's hand in his own, an interpreter standing nearby. Tsuboi stamped his cane emphatically while speaking.

Obama then stepped to Mori and shook his hand. He bowed his head briefly and nodded as the man spoke. He patted Mori on the back and hugged him as the survivor shed a few tears.

The president's interaction with survivors was highly anticipated ahead of his historic visit. Obama did not apologize for the decision to bomb, but paid tribute to the victims and decried the horrors of war.

Obama and Japanese Prime Minister Shinzo Abe then walked along a tree-lined path, past an eternal flame, toward a river that flows by the domed building that many associate with Hiroshima.