Prime Minister Shinzo Abe and foreign delegates will be among those observing a moment of silence at 8:15 AM local time (2315 GMT), when the detonation turned the western Japanese city into an inferno.

An American B-29 bomber named Enola Gay dropped an atomic bomb, dubbed 'Little Boy', on Hiroshima on August 6, 1945, in one of the final chapters of World War II.

Nearly everything around it was incinerated, with the ground level hit by a wall of heat up to 4,000 degrees Celsius -- hot enough to melt steel. About 140,000 people are estimated to have been killed in the attack, including those who survived the bombing itself but died afterwards due to severe radiation exposure.

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On August 9, the port city of Nagasaki was also attacked with an atomic bomb, killing more than 70,000 people.

Japan surrendered days later -- on August 15, 1945 -– bringing the war to a close.

Under-secretary for arms control Rose Gottemoeller will be the most-senior US official sent from Washington to the annual memorial. US ambassador to Japan Caroline Kennedy will also be present at Peace Memorial Park in downtown Hiroshima.     

Opinion remains divided over whether the twin attacks were justified.

While some historians say they prevented many more casualties in a planned land invasion, critics have said the attacks were not necessary to end the war, arguing that Japan was anyway heading for imminent defeat.

Dropping the bombs, which were developed under strict secrecy, was hugely popular with war-weary Americans at the time -- and 70 years on, a majority today still think it was the right thing to do.

Paul Tibbets, who piloted the Enola Gay, said he never had any second thoughts about dropping the bomb, telling a newspaper in an interview in 2002, five years before his death: "I knew we did the right thing".

Washington, which has been a close ally of Tokyo since the war, has never officially apologised for the bombings.

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