Pope Francis said on Wednesday that a string of recent attacks, including the murder of a priest in France, was proof that the "world is at war".

However, speaking to reporters aboard a plane taking him to Poland, the pope said he was not talking about a war of religion, but rather one of domination of peoples and economic interests.

"The word that is being repeated often is insecurity, but the real word is war," he said in brief comments to reporters while flying to southern Poland for a five-day visit.

"Let's recognize it. The world is in a state of war in bits and pieces," he said, adding that the attacks could be seen as another world war, specifically mentioning World War One and Two.

"Now there is this one (war). It is perhaps not organic but it is organized and it is war," he said. "We should not be afraid to speak this truth. The world is at war because it has lost peace."

About 15 minutes later, after greeting journalists individually, Francis took the microphone again and said he wanted "to clarify" that he was not referring to a war of religion.

"Not a war of religion. There is a war of interests. There is a war for money. There is a war for natural resources. There is a war for domination of peoples. This is the war," he said.

"All religions want peace. Others want war. Do you understand?" he said.

He called Jacques Hamel, the priest forced to his knees by Islamist militants on Tuesday who then slit his throat, "a saintly priest", but said he was just one of many innocent victims.

He thanked the many people around the world who have sent their condolences over the killing of Father Hamel, particularly French President Francois Hollande, who spoke to the Pope on Tuesday.