"Men have a short memory," the pope wrote in a letter to the bishop of Bayeux, northern France, near where the landings took place.

For decades, he said, "the memory of the errors of the past" had helped maintain the resolve to "do everything possible to avoid another open world conflict".

But, he added: "I note with sadness that this is no longer the case today and that men have a short memory.

"It is worrying... that the hypothesis of a generalised conflict is sometimes once again being seriously considered -- that people are little by little getting used to this unacceptable possibility."

Pope Francis has regularly denounced the consequences of what he refers to as a "piecemeal Third World War".

France will on Thursday host international leaders at ceremonies to mark the 80th anniversary of the D-Day landings on June 6, 1944, in northern France.

French President Emmanuel Macron will host US President Joe Biden, King Charles III and Canadian Prime Minister Justin Trudeau.

Ukrainian President Volodymyr Zelensky will join the Western leaders in Normandy, northern France, at a ceremony to which no Russian officials have been invited because Moscow invaded Ukraine.