Barack Obama will not apologise for the atomic bombing of Hiroshima when he this week becomes the first sitting US president to visit the city, he told Japanese television.

The comments are the clearest yet from his administration over an issue that raises hackles in the United States and has been the subject of heated debate for decades.

Asked if an apology would be included in remarks he plans to make, he said: "No, because I think that it's important to recognise that in the midst of war, leaders make all kinds of decisions.

"It's a job of historians to ask questions and examine them, but I know as somebody who has now sat in this position for the last seven and a half years, that every leader makes very difficult decisions, particularly during war time."

American airmen launched the world's first atomic strike on Hiroshima on August 6, 1945, causing the deaths of about 140,000 people.

Tens of thousands were killed by the fireball that the powerful nuclear blast generated, with many more succumbing to injuries or illnesses caused by radiation in the weeks, months and years afterwards.

The southern city of Nagasaki was hit by a second bomb three days later, killing 74,000 people, in one of the final acts of World War II.

Obama travelled to Vietnam at the weekend and is due in Japan later this week. He will visit Hiroshima after attending the Group of Seven summit hosted by Japan.

"My purpose is not to simply revisit the past, but to affirm that innocent people die in a war, on all sides, that we should do everything we can to try to promote peace and dialogue around the world, that we should continue to strive for a world without nuclear weapons," Obama said in the interview with NHK, aired Sunday.

US officials have consistently said in the weeks leading up to the visit that there would be no apology.

Obama's upcoming visit has reignited an emotive debate over former US president Harry Truman's epoch-making decision to drop the atomic bombs.

The speed, circumstances and repercussions of Truman's decision remain contentious. In Japan, a majority believe the mass bombing of civilians was unnecessary and perhaps even a crime.

Many Americans believe that it avoided an even bloodier ground invasion of Japan.

Nearly 80 percent of survivors of the atomic bombings are not seeking an apology from Obama, as opposed to 16 percent who want one during Obama's visit, according to a Kyodo News survey of 115 people.

Some thought it best that Japan not seek an apology for fear it would be an obstacle to Obama making the trip to Hiroshima, Kyodo said.

But Terumi Tanaka, the head of a survivors' group, told reporters Friday that survivors want an apology from Obama "to those who died, bereaved families and parents who lost their children".