He's held a meeting with the spiritual leader of the world's Orthodox Christians.
Francis and Patriarch Bartholomew I held a service at the Orthodox Church of St George in Istanbul to mark an important feast day for the Orthodox Church.
Their respective branches of Christianity split in 1054 over differences on the power of the papacy.
The Holy Father used the service to assure the Orthodox faithful unity would not "signify the submission of one to the other, or assimilation".
"I want to assure each one of you gathered here that, to reach the desired goal of full unity, the Catholic Church does not intend to impose any conditions except that of the shared profession of faith," he said.
Patriarch Bartholomew I raised the issue of Islamic State in Iraq: "The modern persecutors of Christians do not ask which church their victims belong to," he said.
"The unity that concerns us is regrettably already occurring in certain regions of the world through the blood of martyrs."