However, he stressed he did not mean a war of religions, but rather a conflict over "interests, money, resources".
He was speaking ahead of his visit to Poland to reporters seeking his comments on the murder of a Catholic priest by French jihadists on Tuesday.
Father Jacques Hamel was killed at a morning mass in his church in Saint-Etienne-du-Rouvray, northern France.
The killing was the latest of a spate of attacks carried out in France and Germany over the past few days and weeks, many of them attributed to Islamist militants.
"The word we hear a lot is insecurity, but the real word is war," the pontiff said.
"We must not be afraid to say the truth, the world is at war because it has lost peace.
"When I speak of war I speak of wars over interests, money, resources, not religion. All religions want peace, it's the others who want war," Pope Francis added.
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