"President Putin, I don't think he truly understands what nuclear weapons are for human beings," said Tanaka, the 92-year-old co-chair of Nihon Hidankyo and a survivor of the 1945 atomic bombing of Nagasaki.

"Nuclear weapons are things which must never be used," he said.

"The use of nuclear weapons ... would be against humanity."

Tanaka was addressing a press conference in Oslo a day before he was due to accept the 2024 Nobel Peace Prize awarded to Nihon Hidankyo at a formal ceremony in Oslo.

Putin began issuing nuclear threats shortly after invading Ukraine in 2022 and signed a decree in late November lowering the threshold for using atomic weapons.

Nihon Hidankyo, a grassroots anti-nuclear organisation, was established in 1956 and is the only nationwide organisation of atomic bomb survivors, who are known as hibakusha.

Around 140,000 people were killed in Hiroshima when the United States detonated an atomic bomb over the Japanese city on August 6, 1945.

A further 74,000 were killed by a US nuclear bomb in Nagasaki three days later

Survivors suffered from radiation sickness and longer-term effects, including elevated risks of cancer.

The bombings were the only times nuclear weapons have been used in history.

The Norwegian Nobel Committee honoured Nihon Hidankyo "for its efforts to achieve a world free of nuclear weapons and for demonstrating through witness testimony that nuclear weapons must never be used again".