As the Pope's three-day trip to Turkey nears its end, a faithful service with Patriarch Bartholomew -- the spiritual head of the world's 300 million Orthodox Christians.

It is here Pope Francis boldly spoke of Islamic militants who, he says, are committing a grave sin in Syria and Iraq.

(SOUNDBITE) (Italian) HEAD OF ROMAN CATHOLIC CHURCH, POPE FRANCIS, SAYING:

"Taking away the peace of a people, committing every act of violence - or consenting to such acts - especially when directed against the weakest and defenseless, is a profoundly grave sin against God, since it means showing contempt for the image of God which is in man."

This is the third time in Turkey that Pope Francis has referred to Islamic State insurgents, who have killed or driven out thousands from Syria and Iraq who do not share their ultra-radical brand of Sunni Islam.

The Pope says inter-religious dialogue is needed to help end these conflicts.

Doing his part, he signed a Joint Decla