HIROSHIMA (AFP) - US President Barack Obama on Friday (May 27) paid moving tribute to victims of the world's first nuclear attack in the western Japanese city of Hiroshima.

"Seventy-one years ago, death fell from the sky and the world was changed," Mr Obama said after laying a wreath, as he became the first sitting US leader to visit the site.

Obama looked sombre as he offered the wreath, lowering his head and pausing for a moment with his eyes closed before withdrawing and watching Japanese Prime Minister Shinzo Abe lay his flowers.

The bomb "demonstrated that mankind possessed the means to destroy itself".

"Why did we come to this place, to Hiroshima? We come to ponder a terrible force unleashed in the not so distant past. We come to mourn the dead," Obama said.

"Their souls speak to us, they ask us to look inward, take stock of who we are," he said.

"Technological progress without equivalent progress in human institutions can doom us. The scientific revolution that led to the splitting of the atom requires a moral revolution as well.

"This is why we come to this place, we stand here, in the middle of this city and force ourselves to imagine the moment the bomb fell.

"We force ourselves to feel the dread of children confused by what they see. We listen to a silent cry."

Obama then greeted ageing survivors, embracing one elderly man who appeared overcome with emotion and shaking hands with another 91-year-old survivor.

Earlier, just hours ahead of his historic visit to Hiroshima, Obama had hailed the "great alliance" between the United States and Japan.

"We are reaffirming one of the greatest alliances in the world between Japan and the United States," he told troops at a base in Iwakuni in the west of the country.

The US has around 47,000 personnel stationed in Japan as part of a security alliance that arose from American occupation in the aftermath of World War II.

"We can never forget that we have to honour all of those who have given everything for our freedom," he told a crowd of uniformed men and women to huge cheers. "I am very proud of you."

Obama was at Iwakuni on his way to Hiroshima, the site of the world's first nuclear attack - a pilgrimage that none of his post-war predecessors have made.

"This is an opportunity to honour the memory of all who were lost in WWII," he said. "It's a testament to how even the most painful divides can be bridged. How two nations can become not just partners but the best of friends."

The trip comes more than seven decades after the Enola Gay bomber dropped its deadly atomic payload, dubbed "Little Boy", over the western Japanese city.

The bombing claimed the lives of 140,000 people, some of whom died immediately in a ball of searing heat, while many succumbed to injuries or radiation-related illnesses in the weeks, months and years afterwards.

A second nuclear bomb was dropped on Nagasaki three days later.

Coming in Obama's final year in office, the visit also marks seven years since he used his trademark soaring rhetoric to call for the elimination of atomic arms in a landmark speech in Prague that helped him win the Nobel Peace Prize.

And while the world today appears no closer to that lofty vision, Obama is expected to use the symbolism of his presence in Hiroshima to highlight a push for peace.

Anticipation was high in Hiroshima, where crowds of Japanese and visiting foreigners gathered near the Hiroshima Peace Memorial Park where Obama was to appear.

"We welcome President Obama," said 80-year-old Toshiyuki Kawamoto. "I hope this historic visit to Hiroshima will push for the movement of abolishing nuclear weapons in the world."

Japanese and American flags flew on the street in front of the site, with a city official saying it was the first time the Stars and Stripes had been raised there.

Obama, who arrived at a US military base west of Hiroshima to address troops after attending a Group of Seven summit elsewhere in Japan, is expected to lay flowers at the cenotaph in the memorial park in Hiroshima and will be accompanied by Japanese Prime Minister Shinzo Abe.

The cenotaph lies in the shadow of a domed building, whose skeleton has been left standing in silent testament to the victims of the first ever nuclear attack.

He told American troops at a base in Iwakuni in the west of the country that visiting Hiroshima was a chance to "honour" the memory of all who died in the war.

"It's a testament to how even the most painful divides can be bridged," he said. "How two nations can become not just partners but the best of friends."

But Obama also said that the two countries were "reaffirming one of the greatest alliances in the world".

Sunao Tsuboi, 91, a Hiroshima survivor, told AFP that he had been invited to the event. He earlier told public broadcaster NHK that if he has the chance to speak with Obama, he would "want to express my gratitude" for his visit.

"I have no intention of asking him for words of apology," said Tsuboi, a long-time anti-nuclear campaigner.

Some quarters of Japanese society, however, have called for such a gesture, though Obama has ruled it out and insisted he will not revisit the decisions of his predecessor Harry Truman at the close of World War II.

While some in Japan feel the attack was a war crime because it targeted civilians, many Americans say it hastened the end of a brutal and bloody conflict, and ultimately saved lives.

The visit, while largely welcomed in Japan, has drawn less sympathetic reactions in other Northeast Asian countries where historical disputes with Tokyo over wartime and colonial aggression remain raw.

In a commentary released late Thursday, North Korea's official KCNA news agency called Obama's trek to Hiroshima an act of "childish political calculation" aimed at disguising the president's true nature as a "nuclear war maniac".

"Obama is seized with the wild ambition to dominate the world by dint of the US nuclear edge," the agency said.

And in Beijing, the government-published China Daily newspaper ran a headline saying: "Atomic bombings of Japan were of its own making."