Pope Francis celebrated Christmas Eve with a late-night Mass in St. Peter's Basilica and a phone call to some Iraqi refugees forced to flee their homes by Muslim militants.

Pope Francis told refugees at the tent camp in Ankawa, a suburb of Irbil in northern Iraq, that they were like Jesus, forced to flee because there was no place for them.

For Christians, Christmas marks the birth of Jesus in a Bethlehem barn manger, chosen because there was no room for his parents at an inn.

"You're like Jesus on this night, and I bless you and am close to you," Francis said. "I embrace you all and wish for you a holy Christmas."

The Ankawa camp houses mostly Christian refugees forced to flee the onslaught by militants of the Islamic State. In a letter to Middle East Christians penned earlier this week, Francis urged them to remain in the region, where Christian communities have existed for 2,000 years, and to help their fellow Muslim citizens present "a more authentic image of Islam" as a religion of peace.

The phone call of Christmas greetings and the night-time Mass kicked off a busy few weeks for the 78-year-old pontiff that includes his traditional Christmas day speech, New Year's Eve vespers, and 2015 greetings a few hours later.

On January 6 he will celebrate Epiphany Mass, and on January 11 he will baptise babies in the Sistine Chapel. A day later he gives his annual foreign policy address to diplomats before boarding a plane for a week-long trip to Sri Lanka and the Philippines.

The Mass comes just days after Pope Francis excoriated the Vatican bureaucracy for a laundry list of sins, including lusting for power and suffering from "spiritual Alzheimer's".

Many of the same cardinals, bishops and priests who received the dressing down were to be on hand in St. Peter's Basilica for the Mass. Several have spoken out in recent days saying Francis was merely asking them to examine their consciences, as any Jesuit spiritual director would do, and to use the Christmas season to heal.