Pope Francis has said he will have his first meeting with a group of sex abuse victims at the Vatican early next month and said he would show zero tolerance for anyone in the Catholic Church who abused children, including bishops.

"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," he 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, adding that three bishops were currently under investigation.

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Francis said he would meet with eight victims and Cardinal Sean Patrick O'Malley of Boston, who is head of a commission set up to study ways of dealing with the crisis.

Speaking to reporters for nearly an hour on the plane taking him back from a visit to the Middle East, the Pope looked alert despite the gruelling three-day trip.

The 77-year-old pontiff fielded questions on a range of topics, including Vatican finances, priestly celibacy, his concern for the environment, and whether he himself would one day retire like his predecessor.

Francis said the victims, several from Europe, would attend his morning Mass and then he would meet them.

O'Malley said last month that the commission he heads will recommend that negligent clerics be held accountable regardless of their rank in the Church.