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.

There is no chance the US administration would take any action against Tibbets if he were found guilty, acknowledged Toshiyuki Tanaka, a group member and war crimes professor at Hiroshima City University's peace institute.

"But our aim is to spur movement towards abolishing nuclear weapons," he said.

He argued that the tribunal could lead Americans to alter policy and said the group was considering an anti-war symposium in Washington to coincide with the verdict.

The Hiroshima bombing killed around 140,000 people -- almost half the city population of the time -- immediately, or in the months afterward from radiation injuries or horrific burns.

It was followed by the dropping of a second atomic bomb three days later in Nagasaki which left more than 70,000 people dead.

Japan has long campaigned against atomic weapons, arguing that it had a special world role as the only nation to suffer a nuclear attack.

In the United States, however, the debate is much more controversial, with many believing the atomic bombing brought an early end to the war.

The campaigners said the tribunal was needed 60 years later as the number of bomb survivors was rapidly diminishing.

"Therefore there is an urgent need for us to closely examine the criminality of the use of nuclear arms," the statement said.

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