Obama will become the first sitting US president to visit Hiroshima, where the first atomic bomb was dropped on August 6, 1945, killing about 140,000 people in total.
Tens of thousands were killed by the fireball that the powerful Hiroshima blast generated, with many more succumbing to injuries or illnesses caused by radiation in the weeks, months and years afterwards.
Vast swathes of the city, including many of its military and industrial installations, were flattened.
"I urge him to apologise to those who died, bereaved families and parents who lost their children," Terumi Tanaka, secretary general of the Japan Confederation of A- and H-Bomb Sufferers Organizations and a survivor of the Nagasaki explosion, told reporters.
Also Read
The comments came as Obama's much anticipated journey to Hiroshima reignited an emotional debate over whether the bombings were justified and whether the US president should apologise.
Obama, who will be in Japan for a Group of Seven summit, will visit Hiroshima's Peace Memorial Park on May 27 and be accompanied by Japanese Prime Minister Shinzo Abe.
"Many atomic bomb victims think it's not all right if (Obama) doesn't apologise," said Toshiki Fujimori, the group's assistant secretary general and also a survivor of the Hiroshima bomb.
"Atomic bomb victims are demanding that it be made clear that the dropping of atomic bombs was inhumane and breaches international law, and that will be confirmed by his apology."
Obama has been vocal on nuclear issues since becoming president in 2009 and made a landmark speech in Prague in April of that year calling for the elimination of nuclear weapons.