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HIROSHIMA, Japan — Barack Obama called for a world without nuclear weapons on Friday as he became the first sitting U.S. president to visit the site of the Hiroshima atomic bombing.

Some 140,000 people were killed when the U.S. dropped an atomic bomb on the city on Aug. 6, 1945.

A helicopter and motorcade brought Obama to the Hiroshima Peace Park Memorial, where he spent a short time in the site's museum and then solemnly placed a wreath at the arched monument.

Obama reflected on the day "death fell from the sky and the world was changed," telling a gathering of survivors and officials that a "wall of fire destroyed a city and demonstrated that mankind possessed the means to destroy itself."

President Barack Obama lays a wreath at Hiroshima Peace Memorial Park on Friday. Shuji Kajiyama / AP

Obama did not apologize for the U.S. actions and instead paid tribute to "all the innocents killed across the arc of that terrible war," saying that "their souls speak to us" and "mere words cannot give voice to such suffering."

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The president evoked the horrors of the war to lay out his vision of a world without nuclear weapons.

"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," Obama said. "We are not bound by genetic code to repeat mistakes of the past — we can learn."

The words he inscribed in the memorial site's guest book echoed that message:

"We have known the agony of war. Let us now find the courage, together, to spread peace, and pursue a world without nuclear weapons," Obama wrote.

Before visiting the memorial Obama met with U.S. servicemen and their families at Marine Corps Air Station Iwakuni in Hiroshima. He was joined by U.S. Ambassador to Japan Caroline Kennedy.

He called the servicemen as the "backbone of our alliance" and told the troops his visit to Hiroshima was an opportunity to "honor the memory" of all those who were lost in World War II.

"It’s a testament to how even the most painful divides can be bridged," Obama said. "How our two nations — former adversaries — cannot just become partners, but become the best of friends and the strongest of allies."

The White House had stressed Obama would not apologize for America's use of the bombs in brief remarks at the Hiroshima Peace Memorial Park.

While an apology would please some in Japan, it would undoubtedly alienate Americans back home — especially giving the trip's timing just ahead of Memorial Day — who credit the bomb with ending the war.

Several survivors of the bombing have said they're not looking for an apology — just change.

Related: Obama's Visit to Hiroshima Stirs Mixed Emotions

“The movement to abolish nuclear weapons hasn’t progressed and in order to raise the momentum we’re hoping that President Obama’s visit will do just that,” 73-year-old Kunihiko Iida told NBC News.

Calling the atomic bombing "an inflection point in modern history," Obama earlier said his visit to the memorial will underscore "the sense of urgency that we should all have" toward nuclear disarmament.

"It's not only a reminder of the terrible toll of World War II and the death of innocents across continents but it's also to remind ourselves that the job's not done," he said Thursday.