Barack Obama has become the first serving US president to visit the Japanese city of Hiroshima, upon which US forces dropped the first of two atomic bombs in the final days of World War Two - killing at least 140,000 people.

"Death fell from the sky and the world was changed," Obama said, after laying a wreath, closing his eyes and briefly bowing his head before an arched monument in Hiroshima's Peace Memorial Park that honors those killed on Aug. 6, 1945.

The bombing, Obama said, "demonstrated that mankind possessed the means to destroy itself."

As he and Japanese Prime Minister Shinzo Abe stood near an iconic bombed-out domed building, Obama acknowledged the devastating toll of war and urged the world to do better.

"We stand here in the middle of this city and force ourselves to imagine the moment the bomb fell ... we listen to a silent cry.

"Mere words cannot give voice to such suffering, but we have a shared responsibility to look directly into the eye of history and ask what we must do differently to curb such suffering again."

The US President spoke of the value of peace, and how important it is for it to be given to every child on earth.

"That is a future we can choose. A future in which Hiroshima and Nagasaki are known not as the dawn of atomic warfare, but as the start of our own moral awakening."

A second atomic bomb, dropped on Nagasaki three days later Hiroshima, killed 70,000 more.