The Japanese city of Hiroshima, reduced to ashes by a US nuclear bomb in 1945, holds its annual commemoration of that attack on Friday, but this will be the first year that a US representative will take part.
US officials announced that Ambassador John Roos would for the first time be among those marking the 65th anniversary of the bombing in the "Peace Memorial Park" -- near "ground zero", the spot where the bomb detonated in the western Japanese city.
Also taking part will be U.N. Secretary General Ban Ki-moon -- the first head of the world body to do so.
Ban is also expected to back calls by a growing number of Japanese for President Barack Obama to visit Hiroshima and Nagasaki, the two cities struck by nuclear attacks in 1945.
The atomic bomb was dropped by the US B-29 warplane Enola Gay on Hiroshima on Aug. 6, 1945, setting the city ablaze and killing thousands instantly. By the end of the year, the toll had risen to 140,000 out of an estimated population of 350,000. Thousands died later of illness and injuries.
Three days after the Hiroshima attack, on Aug. 9, the United States dropped another atomic bomb on Nagasaki, southern Japan. Japan surrendered six days later.
Hiroshima survivor Akihiro Takahashi, 79 and a former head of Hiroshima's Peace Memorial Museum, has been writing to Obama for almost two years, asking him to visit.
"I admire his position of aiming to abolish nuclear weapons," Takahashi, said as he lay in hospital after breaking a bone.
"Some people want to ask for apology, but I do not. I think there will be no peace where there is hatred."
Takahashi still does not have the use of his right hand because of burns suffered in the 1945 attack.
CALL TO DISARMAMENT
On the eve of the ceremony, U.N. Secretary General Ban called for new efforts to rid the world of nuclear arms.
"The only way to ensure (nuclear) weapons will never again be used is to eliminate them all," Kyodo news agency quoted him as saying after meeting attack survivors in Nagasaki.
The issue of Obama visiting the two cities has gained momentum after he expressed a desire to do so last year.
Obama has said the United States, as the only nuclear power to have used a nuclear weapon, has a moral responsibility to act to do away with nuclear arms. The president won the Nobel Peace Prize last year for giving the world "hope for a better future".
Defenders of the bombings say they helped end the war in the Pacific swiftly and spared American lives that would have been lost in an invasion of the Japanese mainland.
No US president has come to Hiroshima or Nagasaki when in office and some experts say that a visit by Obama could create momentum to push forward the nuclear abolition agenda.
"That would be a wonderful opportunity to really jump start this process more aggressively than he (Obama) has," said Peter Kuznick, a professor at American University in Washington.
But the trip could also turn into a domestic gamble for the 49-year-old US leader, as he could be seen as sympathising with Japan. Experts say an apology is out of the question.
"The visit will provide fodder for the administration's domestic critics, who will try to frame it as apology diplomacy," said Daniel M. Kliman of the Center for a New American Security, a liberal thinktank in Washington.
Some experts say a presidential visit to the cities could consolidate the 50-year-old security alliance between Washington and Tokyo, given that ties have been affected by a row over the moving of a US airbase in southern Japan.
But prospects for a visit are further clouded by the hotly contested November US mid-term election and debate in Washington over ratification of a strategic arms deal with Russia.
Hiroshima residents suggested that Obama, who will attend an an Asia-Pacific leaders' summit in Japan in November, would be welcomed by many in their city.
"President Obama won the Nobel Prize, so I want him to do something that matches that status," said Sadae Kasaoka, 77, who lost her parents in the bombing.