Pope Francis attends a prayer calling for peace in Syria, in Saint Peter's square at the Vatican September 7, 2013. Reuters Pope Francis, starting a trip to Poland overshadowed by the killing of an elderly priest in France by suspected Islamist militants, said on Wednesday this and a string of other attacks were proof the "world is at war."

However, speaking to reporters aboard the plane taking him to Krakow for the start of an Catholic youth jamboree, he said he was not talking about a war of religion, but rather one of domination of peoples and economic interests.

After his arrival under heavy security in Krakow, the pope also took on Poland's conservative government, implicitly criticizing its anti-immigration stance.

But his strongest words of the day came while talking to reporters about the killing of Father Jacques Hamel, who on Tuesday was forced to his knees by suspected militants who then slit his throat.

"The word that is being repeated often is insecurity, but the real word is war," he said.

"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. We should not be afraid to speak this truth. The world is at war because it has lost peace."

About 15 minutes later, after an adviser spoke to him, Francis took the microphone again as he was leaving the journalists' section in the plane 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?"