A victim of child sexual abuse by clergy criticised Pope Francis on Saturday for appearing to endorse parents who spanked their children.

"Children don't need to be hit. We need to talk about positive parenting ... physical violence has no part in modern-day child upbringing," said Peter Saunders, who is advising the Vatican on how to deal with its abuse crisis.

"I was hit throughout my childhood and it did me a lot of harm," said Saunders, who was abused by a priest when he was a teenager and is one of 17 members of a Vatican commission tasked with recommending reforms in the 1.2 billion-member Church.

During a talk on fatherhood at his general audience last Wednesday, Francis departed from his prepared text to recount a conversation he once had with a father at a family encounter.

He quoted the man as telling him: "I sometimes have to hit the children a bit but I never slap them in the face so as not to demoralise them." The Pope then added in his own words: "How beautiful! He has a sense of dignity. He must punish. He does it in the right way and then moves on."

The Pope came under heavy criticism in social media for the comments.

Speaking at a news conference on the Catholic Church's sexual abuse crisis, Saunders said he was taken aback when he heard about what he called "the apparent endorsement of hitting children". He added that he planned to talk to the Pope about it when they next met.

"He comes up with some howlers and that proves his humanity, but we need to talk to the Pope about this issue because there are millions of children around the world who are physically beaten on a daily basis," he said.

"It might start off as a light tap, but actually the whole idea of hitting children is about inflicting pain," said Saunders, head of Britain's National Association for People Abused in Childhood.

Marie Collins, a victim of sexual abuse by clergy in her native Ireland, said she was also surprised.

"The good thing about the Holy Father is he speaks without thinking, which in many ways I think is good because it is refreshing," she said. "Maybe sometimes he sort of puts his foot in it, but he's an honest man and I respect him for that. It's not an opinion I would hold."