Pope Francis has revealed he now wears the cross of an Iraqi priest who was killed for failing to renounce his Christian faith.

The cross was apparently given to the pontiff by another Iraqi priest he met on St Peter's square.

"It is a cross this priest had in his hands when he had his throat cut for refusing to renounce Jesus Christ," the pontiff told a gathering of young monks and nuns. "This cross, I wear it here," he added, indicating his chest.

Francis, who has regularly spoken out on the persecution of Christians in the Middle East, added: "Today we have more martyrs than in the first centuries (after Christ)."

Thousands of refugees have left Iraq and Syria in the past year, but the numbers grew significantly when the Islamic State established their caliphate in 2014. The terrorists drive out Christian populations with threats of conversion to Islam, taxes and death.

On Monday the Pope warned that Islamic State terrorists could be gaining access to Europe as incognito refugees, but urged people to remain open to accepting those fleeing war in Syria.

"The truth is that just [250 miles] from Sicily there is an incredibly cruel terrorist group. So there is a danger of infiltration, this is true," Francis told Portuguese broadcaster Radio Renascenca.