A leading Japanese campaigner against nuclear weapons who survived the world's first atomic bomb attack has died at the age of 96.

Sunao Tsuboi was on his way to college when he was caught in the blast at Hiroshima on 6 August 1945, suffering burns all over his body.

Some 140,000 people were killed and Tsuboi dedicated his life to campaigning to eradicate nuclear arms.

He met Barack Obama on his historic visit to Hiroshima as US president.

They shook hands and chatted for about a minute during the meeting in 2016.

This was the first visit by a US president to the city since the attack, which was launched towards the end of World War Two as the Allies pushed back Japan.

"I was able to convey my thoughts," Tsuboi, who played a leading role in Japan's national organisation for Atomic Bomb and Hydrogen Bomb survivors, said later.

He died of anaemia on Saturday, an official from the association told AFP news agency.

On the day the bomb went off, the engineering student was 20 years old.

"Naked, I tried to run away for about three hours on August 6 but finally could no longer walk," he told AFP.

Picking up a stone he wrote on the ground "Tsuboi dies here" before passing out, only waking up several weeks later.

He was so weak and scarred he had to start his recovery by practising crawling on the floor, he told the Associated Press news agency.

Tsuboi went on to teach mathematics at schools in Japan, telling youngsters about his experiences during the war. Pupils nicknamed him "Mr Pikadon" ("Mr Flash-Boom"), he said in an interview on the Hiroshima Peace media website.

"Never give up" was his advice for anti-nuclear campaigners.

"We must not only mourn the death of a great leader for our cause, but we must also continue in his path, undeterred, and always remember his words," Akira Kawasaki of the International Campaign to Abolish Nuclear Weapons, a coalition of non-government organisations, told Japanese public TV.

Tsuboi developed cancer and other illnesses, spending periods of his working life in hospital being treated for anaemia.

Some 127,000 survivors of the nuclear bomb attacks on Hiroshima and Nagasaki are still alive.

Sunao Tsuboi is survived by two daughters and a son, AP reports.