Pope Francis has ended his historic tour of Iraq, departing by plane from Baghdad after visiting conflict-torn cities, meeting Muslim and Christian leaders and preaching peace and coexistence over war.

Pope Francis said the trip was "a lot more" tiring than past foreign visits.

"I have to confess to you that during this trip I felt a lot more tired than during other ones," he told reporters on plane taking them back to Rome. 

During Pope Francis's trip, the first ever papal visit to Iraq, he toured four cities, including Mosul, the former stronghold of the so-called Islamic State, where vast areas still lie in ruins, telling Iraqis that "peace is more powerful than war".

He said Iraq would "always remain with me, in my heart".

The pontiff also made a historical first in meeting Iraq's Grand Ayatollah Ali al-Sistani, Iraq's top Shia Muslim cleric.

He later told journalists that that the cleric was "a wise man", adding: "That meeting was good for my soul."

Iraqis welcomed the Pope and said it was a chance for the world to see their perpetually crisis-hit nation in a new light.

Iraq suffers from chronic mismanagement and corruption, and a steady level of violence often linked to the region's US-Iran rivalry 18 years after the US invaded.