Pope Francis has said that the Catholic Church had "arrived late" in dealing with the problem of sexual abuse of children by priests.

The pope, speaking in unscripted remarks to a commission advising him on how to root out sexual abuse, also acknowledged that early in his papacy he had made one bad call in being too lenient with an Italian priest who later went on to abuse again.

He also said he had decided to change current procedures for dealing with abusive priests by eliminating appeals trials in cases where there was definitive proof.

The pope surprised members of the commission by putting aside his entire prepared speech and chatting to them.

"There is the reality that the Church arrived at the consciousness of these crimes a bit late," he said.

He added: "When consciousness arrives late, the means to resolve the problems also arrive late.

"I am aware of this difficulty but it is reality and I say it plainly: We arrived late."

"The old practice of moving people around and not confronting the problem made consciences fall asleep," he said.

He acknowledged that the commission, which was founded in 2014, had to "swim against the tide," a reference to high level defections from its ranks.

Marie Collins, a non-clerical member who was a victim of priestly abuse when she was a child, quit in frustration last March, citing a shameful lack of cooperation within the Vatican.

Another, Peter Saunders of Britain, took a leave of absence last year in protest over a lack of progress.

The pope said that everyone had to realise that sexual abuse is "a sickness" with a high probability of relapse.

"That person may repent today ... but may commit it again after two years. We have to put it into our heads that this is a sickness," he said.

He said he would change current Vatican procedures to severely limit chances of appeal for priests convicted of child sexual abuse by church tribunals, saying they often were overly legalistic, allowing for reduced sentences on procedural grounds.

"I have decided to balance this out and say that if an abuse of a minor is proven, it is sufficient and there should be no recourse. If the proof is there. Period. It's definitive," he said.

Pope Francis, who was elected in 2013, acknowledged he made one bad judgement early in his papacy concerning an Italian priest, Mauro Inzoli.

In that case, the bishop of Crema, had ruled that Inzoli would be removed from the public ministry while remaining a priest but a Church tribunal ruled that he be defrocked. The pope sided with the bishop.

"I was new [in the papacy] I did not understand these things well and chose the more benevolent of the two sentences but after two years the priest had a relapse. I learned from this," he said.