Pope Francis inTurkey

Francis is expected to use his opening speeches Friday to denounce the violence being committed in God's name by the extremists, and to express solidarity with the Christians and other religious minorities who have been targeted by the onslaught, massacred or forced to leave their homes.

The three-day visit will also give Francis a chance to reach out to Turkey's tiny Christian community - less than 1 percent of Turks are Catholic - and visit with the spiritual leader of the world's Orthodox Christians, Ecumenical Patriarch Bartholomew I.

Francis will tour two of Istanbul's most impressive sites, the Hagia Sofia complex - the Byzantine church-turned-mosque that is now a museum - and the nearby Sultan Ahmet mosque, Turkey's most important place of Muslim worship.

Security will be tight: Turkish media reports said some 2,700 police officers would be on duty during the Ankara leg of the trip alone, and that a court had issued an order allowing police to stop and search cars and carry out random identity checks on people along routes used by the pope.

On the eve of his trip, Francis repeated that it was legitimate to use force to stop the Islamic State advance, but only with the endorsement of the international community. Asked whether dialogue was even possible with a group that is targeting religious minorities, Francis said "Maybe you can't have a dialogue, but you must never close the door."

Francis will wade into some local controversy when he is received Friday by Turkish President Recep Tayyip Erdogan at his huge new palace in Ankara, a 1,000-room complex on once-protected farmland and forest that dwarfs the White House and other European government palaces.

Francis, whose spartan living conditions are well-known, will spend Friday afternoon at the $620 million White Palace, meeting with the president, prime minister and delivering a speech to Turkish dignitaries and diplomatic corps.

The Vatican has dismissed a request by the Ankara branch of the Turkish Chamber of Architects to boycott the meeting, saying Francis would be received wherever the government chose to receive him.