Nuclear weapons are not compatible with the flourishing of peace, Pope Francis said in a letter to Hiroshima to mark the 75th anniversary of the release of an atomic bomb over the city.

"It has never been clearer that, for peace to flourish, all people need to lay down the weapons of war, and especially the most powerful and destructive of weapons: nuclear arms that can cripple and destroy whole cities, whole countries," Pope Francis said in a letter to the governor of Hiroshima, Hidehiko Yuzaki.

The year 2020 marks 75 years since the 1945 nuclear attacks on Hiroshima and Nagasaki by the United States, which resulted in the deaths of hundreds of thousands of Japanese.

Pope Francis visited the hypocenters of the bombings in Nagasaki and Hiroshima during his apostolic visit to Japan in November 2019.

He said his visit to Hiroshima's Peace Memorial and to Hypocenter Park in Nagasaki allowed him to reflect "on the destruction of human life and property wrought in these two cities during those terrible days of war three quarters of a century ago."

"Just as I came to Japan as a pilgrim of peace last year, so I continue to hold in my heart the longing of the peoples of our time, especially of young people, who thirst for peace and make sacrifices for peace," the pope said.

"I carry too the cry of the poor, who are always among the first victims of violence and conflict," he added.

In his letter, Francis repeated his words in Nagasaki in 2019, that "the use of atomic energy for purposes of war is immoral."

"May the prophetic voices of the hibakusha survivors of Hiroshima and Nagasaki continue to serve as a warning to us and for coming generations!" he said.

"To them, and to all who work for reconciliation, we make the words of the psalmist our own: 'For love of my brethren and friends, I say: Peace upon you!.'"

Pope Francis has several times condemned the use of nuclear weapons, including in a video message to Japan ahead of his 2019 visit.

Calling use of the weapons "immoral," he said he was praying they will never be used again.

Japan "is very aware of the suffering caused by war," the pope said. "Together with you, I pray that the destructive power of nuclear weapons will never be unleashed again in human history. Using nuclear weapons is immoral."