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.
‘Best of friends’
Television footage showed Mr Obama arriving and entering the Hiroshima Peace Memorial Museum.
He then walked to the Peace Memorial Park, accompanied by Prime Minister Shinzo Abe. Both men stood in front of the eternal flame.
Mr Obama laid his wreath first, followed by Mr Abe.
Mr Obama earlier 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”.
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.”
‘Just listen’ – Japan’s media on the visit
The Chugoku Shimbun urges Mr Obama to “hear the voices of Hiroshima”. “The people of Hiroshima will be watching the president closely, eyeing to what extent he is truly resolved to advance the abolition of nuclear arms,” it said.
The Asahi Shimbun carries an article saying Mr Obama’s “gestures will shape the visit”, with the “most powerful gesture” being to “just listen to the bomb victims’ memories of suffering and activism”.
The Japan Times says: “To truly pay homage to those whose lives were lost or irrevocably altered by the Hiroshima and Nagasaki bombings, Obama’s visit must galvanise the international community to move without delay toward a world free of nuclear weapons. The fact that these weapons have not been used over the past 70 years does not guarantee a risk-free future for our children.”
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.
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.