Pope Francis made his first public plea for forgiveness on Friday for the "evil" committed by priests who molested children, using some of his strongest words yet on the Roman Catholic Church's sexual abuse crisis.

The Argentine-born pontiff said the Church, which last month named a high-level group on the scandal including an abuse victim, had to take an even stronger stand than before against the scandal that has haunted it for over two decades.

"I feel compelled to personally take on all the evil that some priests — quite a few in number, (although) obviously not compared to the number of all priests — to personally ask for forgiveness for the damage they have done for having sexually abused children," he told members of members of the International Catholic Child Bureau.

"The Church is aware of this ... personal, moral damage carried out by men of the Church, and we will not take one step backward with regards to how we will deal with this problem, and to the sanctions that must be imposed.

"On the contrary, we have to be even stronger. Because you cannot interfere with children," Francis said.

Victims' groups have criticised Francis in recent months for not taking a bold enough stand on the issue and for not meeting with sexual abuse victims in Italy and in a July trip to Brazil.

The Vatican announced in December the creation of a new dedicated group to help the Church fight the abuse crisis but only named its members in late March.

The group of clerics and lay people includes Marie Collins, a survivor of abuse in Ireland in the 1960s who has campaigned for the protection of children and for justice for children who were molested.