Pope Francis said on Sunday that it was wrong to identify Islam with violence and that social injustice and idolatry of money were among the prime causes of terrorism.

"I think it is not right to identity Islam with violence," he told reporters aboard the plane taking him back to Rome after a five-day trip to Poland. "This is not right and this is not true."

Francis was responding to a question about the killing on July 26 of an 85-year-old Roman Catholic priest by knife-wielding attackers who burst into a church service in western France, forced the priest to his knees and slit his throat. The attack was claimed by Islamic State.

"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 baptized 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 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 center 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.

When he started the trip on Wednesday, Francis said the killing of the priest and a string of string of other attacks were proof the "world is at war" but that it was not caused by religion.

He told reporters on the plane that lack of economic opportunities for young people in Europe was also to blame for terrorism.

"I ask myself how many young people that we Europeans have left devoid of ideals, who do not have work. Then they turn to drugs and alcohol or enlist in ISIS," he said, referring to the group also known as Islamic State.