Pope Francis has told victims of sex abuse by Catholic clerics that the church should "weep and make reparation" for crimes he said had taken on the dimensions of a "sacrilegious cult".

The Pope made the remarks while addressing victims of sexual abuses, in the first such meeting since he was elected 16 months ago.

"For some time now I have felt in my heart deep pain and suffering," he said in his strongest comments yet on the crimes, delivered at a mass with adult victims.

"So much time hidden, camouflaged with a complicity that cannot be explained until someone realised that Jesus was looking."

Pope Francis delivered his homily to six victims of abuse, two each from Ireland, Britain and Germany, before meeting each one individually in a gathering that lasted nearly four hours, spending about 30 minutes with each one.

"I ask for the grace to weep, the grace for the church to weep and make reparation for her sons and daughters who betrayed their mission, who abused innocent persons," he said, according to a Vatican transcript.

"Before God and his people I express my sorrow for the sins and grave crimes of clerical sexual abuse committed against you. And I humbly ask forgiveness."

Sexual abuse scandals have haunted the Catholic Church for over two decades but became a major issue in the United States about 10 years ago.

Since then they have cast a shadow over local churches in Ireland, Germany, Belgium, the Netherlands and other countries and badly tarnished the church's image.

The Pope told the victims that he realised that they and others had suffered "often unrelenting emotional and spiritual pain, and even despair" and that some had turned to drugs or even taken their own lives.

"The deaths of these so beloved children of God weigh upon the heart and my conscience and that of the whole church," he said.

The meeting was closed to media but the Vatican spokesman, who participated, said it was "very intense and moving".

"If you see the persons that come out of this meeting with the Pope, you will understand it's not a public relations event," Father Federico Lombardi said.

"It is a very profound, spiritual encounter, dialogue with a father, a person who tries to understand deeply."

The names and ages of the victims were not released but they were believed to be in their 30s and 40s, according to people who helped organise the gathering.

Victims groups urge Pope to take further action

Victims groups welcomed the meeting but said it should have taken place long ago.

"I think it's very important that the Pope meet with victims," said Anne Doyle of BishopsAccountability.org, a US-based documentation centre on abuse in the Catholic Church.

"We know this Pope is capable of compassion and his refusal to meet with sexual abuse victims so far has been inconsistent with the mercy he has shown with so many marginalised. This is something that he had to rectify."

Ms Doyle said the Pope should quickly follow up with "several core actions" to show that the meeting is not merely ceremonial.

"He definitely must explicitly tell his bishops that all Church officials must report crimes and suspected crimes to civil authorities," she said, pointing that in a number of developing countries it is up to the victim to report sexual crimes.

In many cases of abuse, most of which took place decades ago but surfaced in the past 15 years or so, bishops seeking to protect the church's reputation moved priests from parish to parish instead of defrocking them or handing them over to police.

Victims' groups have pressed the Vatican to hold bishops who either shielded abusers or were negligent in protecting children to account along with abusers themselves.

Pope Francis said he would not tolerate abusers and bishops would be held accountable if they shielded them.

In February, the United Nations Committee on the Rights of the Child accused the Vatican of systematically turning a blind eye to decades of abuse and attempting to cover up sex crimes.

The Vatican called the report unfair and ideologically slanted.

The Vatican says 3,420 credible accusations of sexual abuse by priests had been referred to the Vatican in the past 10 years and 824 clerics defrocked.

ABC/Reuters