Pope Francis has issued a card showing a 1945 photo of victims of the Nagasaki nuclear bombing in a New Year warning over the threat of global conflict and its effect on humanity.

The Vatican is to print and distribute copies of the image, which shows a young Japanese boy carrying his dead brother on his shoulders to a cemetery, with the words "the fruit of war" written on the back.

The picture was taken by U.S. Marine photographer Joe O'Donnell shortly after American forces dropped atomic bombs on Japan, first on Hiroshima and then days later on Nagasaki at the end of the Second World War.

The cards issued by Francis will carry his own signature along with a caption: "The young boy's sadness is expressed only in his gesture of biting his lips which are oozing blood," The Independent newspaper in the UK reported.

O'Donnell spent four years documenting the trail of destruction after the bombing of Nagasaki and Hiroshima, the first and only times an atomic bomb has been used in a wartime act. 

The Vatican press circulated the photo ahead of the Catholic Church's World Day of Peace, which is celebrated every Jan.1 January.

Vatican analyst John Allen said, "The gesture is consistent with Francis's effort since his election to speak out against what he describes as a 'Third World War' today, being fought in piecemeal fashion in various parts of the world."

"The pontiff has also spoken about the disproportionate suffering children often experience in conflicts, including the risk of being enrolled as child soldiers," said Allen writing in Crux.

"The gesture with the photo of Nagasaki also comes at the close of a year in which the threat of nuclear conflict once again had the world on edge, in light of North Korea's threat to use nuclear weapons, and U.S. President Donald Trump's vow that America would unleash 'fire and fury' should that happen."