Barack Obama is set to arrive in Hiroshima and become the first serving US president to visit the Japanese city since the 1945 nuclear bombing.
Mr Obama flew into a US base nearby, after leaving the G7 summit.
Mr Obama has said he will not be issuing an apology for the nuclear attack, but will honour all those who died in World War Two.
He told Japanese media the visit would show that “even former adversaries can become the strongest of allies”.
“Hiroshima reminds us that war, no matter the cause or countries involved, results in tremendous suffering and loss, especially for innocent civilians,” he wrote in the Asahi newspaper.
The world’s first nuclear bomb attack, on Hiroshima on 6 August 1945, killed at least 140,000 people. Two days later a second nuclear bomb was dropped on Nagasaki, killing another 74,000.
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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.”
The BBC’s John Sudworth in Hiroshima says 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.
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.
Seventy years since Hiroshima
- 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
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