Pope Francis said ISIS militants were carrying out a "profoundly grave sin against God" in Syria and Iraq, calling on Sunday for dialogue and action against poverty to help end conflicts there.
The pope spoke on the last day of his weekend trip to Turkey, which is sheltering nearly 2 million refugees from Syria, thousands of Christians among them.
At a joint service with Patriarch Bartholomew I, spiritual head of the world's 300 million Orthodox Christians, Francis said people of all faiths could not remain indifferent to the cries of the victims of the "inhumane and brutal" war next door.
"Taking away the peace of a people, committing every act of violence - or consenting to such acts - especially when directed against the weakest and defenceless, is a profoundly grave sin against God," he said during the service.
The pope also condemned Friday's attack on Muslim worshippers at the main mosque in northern Nigeria's biggest city, Kano, in which at least 81 people were killed.
It was the third time in as many days that the pope referred to ISIS militants, who have killed or driven many who do not share their ultra-radical views out of swathes of Syria and Iraq.
The pope said stopping poverty was key partly because it gives rise to "the recruitment of terrorists". He has said that while it is lawful for the international community to use force to stop an "unjust aggressor," a lasting solution must be found.
In a separate joint statement issued during the service at the Ecumenical Patriarchate, the official seat of the Orthodox Christian patriarch, Francis and Bartholomew said:
"Muslims and Christians are called to work together for the sake of justice, peace and respect for the dignity and rights of every person, especially in those regions where they once lived for centuries in peaceful coexistence and now tragically suffer together the horrors of war."
The leader of the world's 1.2 billion Roman Catholics prayed in Istanbul's most famous mosque on Saturday and has used the trip to reach out to Muslims to oppose the conflict in neighbouring countries.
Father Claudio Monge, an Italian Catholic priest in Turkey for 12 years who is involved in Christian-Muslim dialogue said: "The message is that those who use God as a pretext for violence cannot be a true believers. Those who have ears to hear will understand. He is appealing to true believers."