Outspoken evangelist Franklin Graham has targeted Pope Francis for saying recent terrorist violence was not religiously motivated.

The pontiff told reporters en route to Poland where thousands had gathered for World Youth Day that "the world is at war" but stressed it was not a religious war.

"It's war, we don't have to be afraid to say this," he said on board the papal plane. But he late stressed it was "a war of interests, for money, resources. ... I am not speaking of a war of religions. Religions don't want war. The others want war."

The controversial son of world renowned evangelist Billy Graham, Franklin criticised the Pope and said "we should call it what it is" in a post on his Facebook page on Sunday.

"I agree that the world is at war—but I disagree that it's not a war of religion," he wrote.

"It is most certainly a war of religion. Religion is behind the violence and jihad we're seeing in Europe, the Middle East, Asia, and here in this country.

"It's a religion that calls for the extermination of "infidels" outside their faith, specifically Jews and Christians. It's a religion that calls on its soldiers to shout "Allahu Akbar" ("God is Great" in Arabic) as they behead, rape, and murder in the name of Islam."

On his return flight Pope Francis caused further controversy when told reporters it was wrong to associate Islam with violence.

"I think that in nearly all religions there is a always a small fundamentalist group," he said, adding

"We have them," referring to Catholicism.

"I don't like to talk about Islamic violence because every day when I look at the papers I see violence here in Italy – someone killing his girlfriend, someone killing his mother-in-law. These are baptised Catholics," he said.

"If I speak of Islamic violence, I have to speak of Catholic violence. Not all Muslims are violent," he said.

He said there were various causes of terrorism.

"I know it is dangerous to say this but terrorism grows when there is no other option and when money is made a god and it, instead of the person, is put at the centre of the world economy," he said.

"That is the first form of terrorism. That is a basic terrorism against all humanity. Let's talk about that," he said.