Barack Obama became the first sitting US President to visit Hiroshima more than seven decades after the United States dropped the first wartime atom bomb on the city.

"We come to ponder the terrible force unleashed in the not so distant past," he said in a 20-minute address at Hiroshima Peace Park, where he and Shinzo Abe, the Japanese Prime Minister, each laid a wreath.

"We come to mourn the dead, including 100,000 Japanese men, women and children, thousands of Koreans and a dozen Americans held prisoner. Their souls speak to us."

As anticipated, he did not offer an apology for the deaths.

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The President met a number of survivors of the 1945 bombing, among them Sunao Tsuboi, 91, and Shigeaki Mori, 79, who shed tears as they embraced.

The visit took place seven years after he made a rousing plea for an end to atomic weapons during a speech in Prague which helped him to win the Nobel Peace Prize.

Most Americans have long viewed the atomic bombing of Hiroshima and Nagasaki a few days later as necessary to end the war. Most Japanese believe they were unjustified.