The violence being carried out by ISIS militants across large sections of Iraq and Syria is a "profoundly grave sin against God", the Pope has said.
Pope Francis was speaking at the Patriarchal Church of St George in Istanbul, Turkey, where he has been on an official visit.
Turkey is hosting around two million Syrian refugees, many of whom have fled the ISIS terror. The group has been shocking in its brutality, executing and raping those who do not abide by its rules or ideology.
Christians and Yazidis are among the victims of the IS militants and thousands have sought shelter in neighbouring countries or provinces.
Pope Francis condemned the violence on Sunday: "Taking away the peace of a people, committing every act of violence, 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."
He suggested the scale of devastation only served to increase the need for reconciliation between the churches.
"The cry of the victims of conflict urges us to move with haste along the path of reconciliation and communion between Catholics and Orthodox," he said.
Pope Francis and Patriarch Bartholomew I of Constantinople issued a joint statement at the end of his visit today in which they spoke of the need for action from the international community and "constructive dialogue" with Muslims.
"The terrible situation of Christians and all those who are suffering in the Middle East calls not only for our constant prayer, but also for an appropriate response on the part of the international community," they said.
