VATICAN CITY--With a warning about “the fruit of war,” Pope Francis issued a card featuring a poignant photo of a boy with his dead brother strapped to his back after the atomic bombing of Nagasaki.

Copies of the card were distributed to Roman Catholic Church officials late last year at the behest of the pope, a gesture that Vatican officials described as “uncommon.”

The photo was taken at a crematorium in Nagasaki by U.S. military photographer Joe O’Donnell shortly after the city was turned into ruins by the atomic bombing on Aug. 9, 1945.

The pope wrote his signature on the back of the card, along with the words “the fruit of war.”

Francis explained the photo in Spanish: “The boy, carrying his dead brother on his shoulders, waited for his turn at the crematory. His grief is shown on his lips, which were caked with blood because he was biting them so hard.”

The pontiff has been calling for a world free of nuclear weapons.

In November, he met participants in a symposium on nuclear disarmament and told them that nuclear weapons are incompatible with world peace.