Barack Obama has become the first serving US president to visit Hiroshima since the World War Two nuclear attack.

Obama said the memory of 6 August 1945 must never fade, but did not apologise for the US attack - the world's first nuclear bombing.

Obama spoke to a number of survivors and in an address called on nations to pursue a world without nuclear weapons.

At least 140,000 people died in Hiroshima and another 74,000 two days later in a second bombing in Nagasaki.

'Best of friends'

Obama first visited the Hiroshima Peace Memorial Museum before walking to the Peace Memorial Park, accompanied by Prime Minister Shinzo Abe. Both men stood in front of the eternal flame.

Obama laid a wreath first, followed by Abe.

Obama had earlier flown into the nearby Iwakuni Marine Corp base nearby, after leaving the G7 summit.

Obama told service personnel at the base: "This is an opportunity to honour the memory of all who were lost during World War Two.

Hiroshima residents say they want President Obama to understand the suffering of victims. Photo: Reuters

"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.

Obama praised the US-Japan alliance as "one of the strongest in the world", with his visit "a testament to how even the most painful divides can be bridged - how our two nations, former adversaries, cannot just become partners, but become the best of friends and the strongest of allies".

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."

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."