Leaders of the Roman Catholic church who failed to "respond adequately" to reports of child sex abuse by paedophile priests caused "even greater suffering" to their victims and will in future be held accountable, Pope Francis has said; in a clear rebuke to bishops who helped cover up the scandal and shield abusers.

In his strongest condemnation yet of the clerical abuse that shook the Catholic community around the world, Francis asked for forgiveness on behalf of the church not only for the perpretrators of abuse but those senior figures whose "sins of omission" he said had exacerbated the problem.

The sexual abuse of minors by priests and other men of the cloth was, he said, was a "crime and grave sin" that required the church to "make reparation". The Argentinian pontiff delivered the powerful homily at a morning mass in the Vatican before a group of six abuse victims, including two from the UK. Sixteen months into his papacy, it was his first such encounter.

"It is something more than despicable actions," Francis said of clerical sex abuse. "It is like a sacrilegious cult, because these boys and girls had been entrusted to the priestly charism in order to be brought to God. And those people sacrificed them to the idol of their own concupiscence."

"There is no place in the Church's ministry for those who commit these abuses, and I commit myself not to tolerate harm done to a minor by any individual, whether a cleric or not."

It is not the first time that Francis has condemned the abuse, but his words delivered at the Santa Martha guesthouse on Vatican grounds were particularly pointed towards those clerics who may have enabled the abuse to be "camouflaged with a complicity". Advocates of abuse victims have long complained that the church has yet to act to punish those bishops who contributed to cover-ups.

"I beg your forgiveness for the sins of omission on the part of church leaders who did not respond adequately to reports of abuse made by family members, as well as by abuse victims themselves. This led to even greater suffering on the part of those who were abused and it endangered other minors who were at risk," said Francis, according to a translation made available by the Vatican.

All bishops, added the pope, must exercise the utmost care in order to protect minors. And they will be held accountable, he warned. After the homily, the pope held private meetings with each of the six abuse victims from 9am until 12.20pm, said Father Federico Lombardi, the Vatican spokesman. He declined to identify the people, saying only they were all adults, two of whom came from tBritain, two from Ireland and two from Germany.

The meetings, said Lombardi, were "extremely broad and intense" and left the victims with "the feeling that they had been listened to … with great attention". However the meeting had been dismissed in advance by many advocates for abuse victims around the world, who said it was no substitute for concrete policy changes.

In May, after the meeting was announced, David Clohessy, executive director of the main US victims' group, Survivors Network of Those Abused by Priests (Snap), told Associated Press: "The simple truth is this is another gesture, another public relations coup, another nice bit of symbolism that will leave no child better off and bring no real reform to a continuing, scandal-ridden church hierarchy."

The Vatican rejected this argument on Monday, with Lombardi saying he was not surprised the advocates could not understand the "positive intentions" of either Francis or Benedict XVI, who met with abuse victims on several occasions. "It is totally clear that it was not a public relations event," he told journalists. "If you [had seen] the people coming out of this meeting with the pope you [would have seen] that this was not a public relations event."

The Holy See has been hauled over the coals repeatedly this year by two United Nations human rights panels, both of which unleaded harsh criticism on the church for its handling of the scandal, and urged it to do more to punish paedophile priests and their protectors, support victims and protect children.