The United States President , Barack Obama has arrived in Hiroshima to become the first serving US president to visit the Japanese city since the 1945 nuclear bombing.
Mr Obama flew into the Iwakuni US base nearby, after leaving the G7 summit.
He said his visit was “a testament to how even the most painful of divides can be bridged”. But he also says he will not be apologising for the attack.
At least 140,000 people died in Hiroshima on 6 August 1945, in what was the world’s first nuclear bombing.
Two days later a second nuclear bomb was dropped on Nagasaki, killing another 74,000.
Mr Obama told service personnel at the Iwakuni Marine Corp base, some 40km (25 miles) from Hiroshima: “This is an opportunity to honour the memory of all who were lost during World War Two.
“It’s a chance to reaffirm our commitment to pursuing the peace and security of a [world] where nuclear weapons would no longer be necessary.”
Mr Obama praised the US-Japan alliance as “one of the strongest in the world”, with his visit showing how “two nations, former adversaries, cannot just become partners, but become the best of friends”.
Mr Obama will lay a wreath at the cenotaph, where an eternal flame remembers Hiroshima’s dead. He will be joined by bomb survivors living in the now thriving city.
Many in the US believe the use of the nuclear bomb, though devastating, was right, because it forced Japan to surrender, bringing an end to World War Two.
The daughter of one survivor, who was visiting the memorial on Friday, said the suffering had “carried on over the generations”.
“That is what I want President Obama to know,” Han Jeong-soon, 58, told the Associated Press. “I want him to understand our sufferings.”
Seiki Sato, whose father was orphaned by the bomb, told the New York Times: “We Japanese did terrible, terrible things all over Asia. That is true. And we Japanese should say we are sorry because we are so ashamed, and we have not apologised sincerely to all these Asian countries. But the dropping of the atomic bomb was completely evil.”
Reports from Hiroshima say there is likely a strategic purpose to the visit, as a symbol of the deepening alliance between Washington and Tokyo in a region wary of China’s rising military might.
Mr Obama referred to this in his speech at the base, saying: “As president, I made sure that the United States is leading again in the Asia Pacific, because this region is vital.”
Jimmy Carter has visited Hiroshima, but after the end of his presidency.
A US ambassador attended the annual commemoration for the first time in 2010.
The bomb was nicknamed “Little Boy” and was thought to have the explosive force of 20,000 tonnes of TNT
Paul Tibbets, a 30-year-old colonel from Illinois, led the mission to drop the atomic bomb on Japan
The Enola Gay, the plane which dropped the bomb, was named in tribute to Col Tibbets’ mother
The final target was decided less than an hour before the bomb was dropped. The good weather conditions over Hiroshima sealed the city’s fate
On detonation, the temperature at the burst-point of the bomb was several million degrees. Thousands of people on the ground were killed or injured instantly.