Some 30 academics, lawyers and peace activists are preparing for the trial to start next year on the 60th anniversary of the bombings, with the verdict likely to be read out in Washington in early 2006.

Defendants could be key US decision-makers including late president Harry Truman and secretary of war Henry Stimson, along with Robert Oppenheimer and other scientists and the military personnel who carried out the order.

"As the statute of limitations is not applicable to war crimes, the responsibility should lie with the present US government, too," the Hiroshima-based group said in a statement.

The group has invited international law experts to act as prosecutors and judges.

The activists said the failure to pursue criminal charges over the bombings in the final days of World War II led to the expansion of nuclear weapons and further wars, such as those seen in Afghanistan and Iraq.

"The bombing of Hiroshima and Nagasaki typifies two kinds of crimes against humanity, indiscriminate bombing and mass killing, both common phenomena in contemporary warfare," the group said.

Citizens of Hiroshima and Nagasaki "bear a moral responsibility to represent the voices" of all victims of indiscriminate bombing throughout the world, the trial's preparatory committee said.

"No national government has ever tried to fulfil its responsibility by pursuing justice on this matter," it said.

Paul Tibbets, pilot of the Enola Gay B-29 which dropped the bomb on Hiroshima in the world's first atomic attack, would be the only person alive among those who could be accused in the tribunal.