Barack Obama paid moving tribute to victims of the first atomic bomb Friday and called for a world free of nuclear weapons, during a historic and emotional visit to Hiroshima.

In a ceremony loaded with symbolism, the first sitting US president to visit the city met survivors of the fearsome attack that marked one of the final, terrifying chapters of World War II.

"71 years ago, death fell from the sky and the world was changed," Obama said of a bomb that "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," he said.

As crows called through the hush of the Hiroshima Peace Memorial Park, Obama offered a floral wreath at the cenotaph, pausing in momentary contemplation with his eyes closed and his head lowered.

The site lies in the shadow of a domed building, whose skeleton stands in silent testament to those who perished.

Japanese Prime Minister Shinzo Abe followed by offering his own wreath and a brief, silent bow.

After both men had spoken, Obama, whose predecessor Harry Truman gave the go-ahead for the world's first nuclear strike, greeted ageing survivors, embracing 79-year-old Shigeaki Mori, who appeared overcome with emotion.

"The president gestured as if he was going to give me a hug, so we hugged," Mori told reporters afterwards.

Obama also chatted with a smiling Sunao Tsuboi, 91, who had earlier said he wanted to tell the US president how grateful he was for his visit.

- Ball of searing heat -

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; others succumbed to injuries or radiation-related illnesses in the weeks, months and years afterwards.

A second nuclear bomb destroyed the city of Nagasaki three days later.

The visit also marks seven years since Obama's memorable speech in Prague in which he called for the elimination of atomic weapons, a call that helped him win the Nobel Peace Prize.

Crowds of young and old gathered to meet the American president, who retains enormous star power in Japan.

"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."

- 'We listen to the silent cry' -

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.

As expected, Obama offered no apology for the bombings, having insisted that he would not revisit decisions made by Truman at the close of a brutal war.

As an eternal flame flickered behind him, however, he said leaders had an obligation to "pursue a world without" nuclear weapons.

"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."

"The world was forever changed here but, today, the children of this city will go through their day in peace," the US president said. "What a precious thing that is."

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

Though there had been calls for an apology, public reaction to the visit and the speech was overwhelmingly positive.

Megu Shimomura, a 14-year-old schoolgirl, one of the selected guests at the ceremony, told AFP: "I was thrilled to attend the historic event. Obama is someone who lives in a very different world than I do but I felt his humanity."

Shinzo Abe praised the "courage" of the visit, which he said offered hope for a nuclear free future.

"An American president comes into contact with the reality of an atomic bombing and renews his resolve toward realising a world without nuclear weapons," he said.

"I sincerely welcome this historic visit, which has long been awaited by not only people of Hiroshima, but by all Japanese people."

The pilgrimage drew a less sympathetic response 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."