A grandson of former US president Harry Truman, who authorised the atomic bombing of Japan during World War II, met survivors in Tokyo Friday, calling it "a good first step towards healing old wounds".

Clifton Truman Daniel, 55, was in Japan to attend 67th anniversary ceremonies of the bombs falling -- on August 6 in Hiroshima and August 9 in Nagasaki -- the first Truman relative to attend the annual events in the southern Japanese cities.

Tens of thousands of people come out every year to remember the 1945 atomic bombing, estimated to have killed more than 200,000 people, either instantly or from burns and radiation sickness following the blast.

Truman's grandson, a former journalist who was invited by an anti-nuclear group, talked with a handful of survivors and students at a Tokyo University forum on Friday.

"The meeting was great," Daniel told reporters following the two-hour conversation during which he mainly listened to survivors who are now in their 70s and 80s.

"The most impressive thing is that survivors and students and all of us can come together and talk, and they can share their stories," said Daniel, who lives in Chicago.

In a separate interview with AFP, Daniel added that "it's a good first step toward healing old wounds. We are looking at this... as a good first step to talk and to better understand each other".

The survivors who attended the forum generally welcomed Daniel's visit. "It's good to meet the grandson of Mr Truman as we have not had a chance to meet him before," said Nobuo Miyake, 83, who survived the Hiroshima bombing and now lives in Tokyo.

Kohei Koba, another male survivor, 79, said: "Since he is a grandson, he has no direct responsibility. Rather, I felt as if I met a distant relative."

But Reiko Yamada, a 77-year-old female survivor, said: "I would like him to know that some of those who lost their family members in the bombings will never forgive (the United States) no matter what."

Masashi Ieshima, another survivor in his 70s, said: "I'm not sure we could have sufficient communication in such a limited time. I hope he will learn about the real tragedy when he visits Hiroshima and Nagasaki."

During his stay in Japan, Daniel is scheduled to hold talks with the mayors of the two cities and meet more survivors of the bombings.

Asked if he plans to publicly express his sympathy to survivors, Daniel said only: "I certainly can feel terrible for what happened to them. It's obviously a difficult subject."

While opinions remain divided over whether atomic bombs were necessary to end the war, Daniel defended his grandfather, who ordered their use after Japan refused to surrender.

"I can't second-guess my grandfather... (but) there is no right decision in war," he said.

"My grandfather always said that he made that decision to end the war quickly. That's what he believed.

"(He) was horrified by the destruction caused by those weapons and dedicated the rest of his presidency trying to make sure that it didn't happen again.

"I hope that I can do the same, to work to hopefully rid the world of nuclear weapons," he said.