Foreign Minister Wang Yi reminds international community of scars left by Nanjing Massacre

US President Barack Obama paid moving tribute on Friday to victims of the world's first nuclear attack, during a historic visit to Hiroshima, but did not apologize for his country's use of the atomic bomb.

In a soaring speech watched by survivors of the atomic blast, Obama said the bomb that rent the city on August 6, 1945 "demonstrated that mankind possessed the means to destroy itself".

"Seventy-one years ago, death fell from the sky and the world was changed," he said after laying a large floral wreath.

Obama greeted aging survivors and embraced one elderly man who appeared overcome with emotion.

He 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.

Obama, wearing a dark suit, looked somber as he offered a wreath at the cenotaph, in the shadow of a wrecked building that stands in silent tribute to the dead.

The president lowered his head and closed his eyes as he paused for a moment's contemplation, before withdrawing and watching Japanese Prime Minister Shinzo Abe do the same.

"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 to the assembled crowd.

"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."

"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."

There was some criticism of Obama as his visit could be interpreted as tantamount to an apology for the nuclear attacks on the two Japanese cities. Many in the United States believe bombings were necessary to bring an earlier end to World War II and save thousands of lives.

While Hiroshima commands the spotlight, Foreign Minister Wang Yi on Friday reminded the international community of the scars left by the massacre in Nanjing in 1937. He said that while Hiroshima is worthy of attention, Nanjing should not be forgotten and deserves even more attention.

On Dec 13, 1937, Japanese troops captured Nanjing, then China's capital, and started a barbarous spree killing that lasted over a month. More than 300,000 Chinese civilians and soldiers who had laid down their arms were murdered and over 20,000 women were raped.

"The victims deserve sympathy," he said, "but the perpetrators could never shake off their responsibility."

On Dec 13 every year since 2014, China marks National Memorial Day for Nanjing Massacre Victims.

Motofumi Asai, former director of China and Mongolia division in Japan's foreign minister, called Obama's Hiroshima trip a showcase of celebration for a stronger Japan-US alliance. For the two allies, Hiroshima, Nagasaki and Pearl Harbor have been the thorns in their relations.

Abe has said he has no plan to visit Pearl Harbor that imperial Japan attacked in 1941. The sneak attack dragged the US into the World War II.

Abe said Obama's Hiroshima visit brought hope to people seeking a nuclear-free world.

"It is a groundless conceit," Asai said. "Obama has not come up with positive policies to reduce nuclear weapons let alone dismantle nuclear weapons in the seven years after he delivered a speech in Prague in 2009."

There is "zero possibility" of concrete initiatives on nuclear disarmament from the Obama administration that is due in months, Asai said.

Cai Hong in Tokyo contributed to this story.

US President Barack Obama delivers a speech at the cenotaph in Hiroshima Peace Memorial Park in Hiroshima on Friday. Johannes Eisele / Agence France Presse