Pope Francis has likened sex abuse by Catholic clergy to a "satanic Mass" - and has vowed to take a "zero tolerance" approach to paedophile priests.

The pontiff, 77, said three bishops were under investigation by the Vatican, although it is not clear if they were accused of committing abuse itself or of having covered it up. 

He was speaking on board the papal plane after visiting Jerusalem, and ahead of a meeting with a group of sex abuse victims at the Vatican next month.

The Argentine-born pope said: "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.

"We must go ahead with zero tolerance."

The head of the Catholic Church has previously asked for forgiveness for the "evil" abuse of children, terming it a "moral damage carried out by men of the Church".