Barack Obama travelled to Hiroshima on Friday, becoming the first sitting US president to visit the western Japanese city where US forces dropped the first atomic bomb in the closing days of World War II.
Obama, accompanied by Japanese Prime Minister Shinzo Abe, laid a wreath at the arch-shaped Hiroshima Peace Memorial near ground zero, which was inscribed with the words, "Let all the souls here rest in peace; For we shall not repeat the evil."
"Seventy-one years ago on a bright cloudless morning, death fell from the sky and the world was changed,” Obama said, referring to the atomic bombing.
The nuclear attack on the city on August 6, 1945, killed tens of thousands of residents instantly, and by the end of the year, some 140,000 in total had died.
"A flash of light and a wall of fire destroyed the city and demonstrated that mankind possessed the means to destroy itself," the president said.
"Why do we come to this place, to Hiroshima?" he asked. "We come to ponder the terrible force unleashed in the not-so-distant past. We come to mourn the dead."
Obama stopped short of offering an apology for the US atomic bombing in the speech, which he delivered after attending this year's G7 summit in the coastal city of Shima.
Some survivors, US Ambassador to Japan Caroline Kennedy, Hiroshima Mayor Kazumi Matsui and Nagasaki Mayor Tomihisa Taue were among about 100 people in the audience at the ceremony.
On Thursday, Obama said he was going to use the trip to Hiroshima to underscore "the very real risks that are out there and the sense of urgency we all should have" about nuclear weapons.
The "job is not done" in terms of reducing the prospect of future attacks, he added.
In 1945, three days after the bombing on Hiroshima, a second atom bomb released from a B-29 was detonated over the port city of Nagasaki. Around 74,000 lost their lives by the end of the year due to that attack.