Pope Francis has branded sexual abuse of children by priests a crime comparable to a "satanic Mass".

He said he would show zero tolerance for anyone in the Catholic Church who abused children, including bishops.

Speaking to reporters on the plane taking him back from a visit to the Middle East, the Pope also announced that he would have his first meeting with a group of abuse victims at the Vatican early next month.

Asked about whether he would move against bishops who were accused of sexual abuse, he said "there will be no daddy's boys" and no privileges, adding that three bishops were currently under investigation.

"Sexual abuse is such an ugly crime ... because a priest who does this betrays the body of the Lord. It is like a satanic Mass," the Pope said, in some of the toughest language he has used on a crisis that has rocked the Church for more than a decade.

"We must go ahead with zero tolerance," he said.

He said he would hold a meeting with around eight victims of sexual abuse at the Vatican early next month.

It would be attended by Cardinal Sean Patrick O'Malley of Boston, who is head of a commission set up to study ways of dealing with the crisis.

The Pontiff, who spoke to reporters for nearly an hour, said the victims, several from Europe, would attend his morning Mass and then he would meet them.

It will be the first time for Pope Francis to meet sexual abuse victims since his election in March 2013.

Pope Francis also said he believed that Catholic priests should be celibate, but that the rule was not an unchangeable dogma, and "the door is always open" to change.

He made similar comments when he was archbishop of Buenos Aires but his remarks to reporters on a plane returning from a Middle East trip were the first he has made since becoming Pope.

"Celibacy is not a dogma," he said in answer to a question about whether the Catholic Church could some day allow priests to marry as they can in some other Christian churches.

"It is a rule of life that I appreciate very much and I think it is a gift for the church but since it is not a dogma, the door is always open," he said.

The Pope said he would be open to retiring like his predecessor, Benedict XVI, instead of ruling for life, adding that the concept of a "pope emeritus" could someday become normal.

"I will do what the Lord tells me to do," he told reporters, when asked if he someday would retire if his health did not permit him to rule the 1.2 billion-member Church properly.

"I think that Benedict XVI is not a unique case. I think we should see him as an institution who opened a door, the door of emeritus popes."