Pope Francis has admitted that clerics have sexually abused nuns, and in one case they were kept as sex slaves.
He said in that case his predecessor, Pope Benedict, was forced to shut down an entire congregation of nuns who were being abused by priests.
It is thought to be the first time Pope Francis has acknowledged the sexual abuse of nuns by the clergy.
He said the Church was attempting to address the problem but said it was "still going on".
Pope Francis made the comments on Tuesday to reporters while on a historic tour of the Middle East.
He admitted that priests and bishops had abused nuns, but said the Church was aware of the issue and "working on it".
"It's a path that we've been on," he said.
"Pope Benedict had the courage to dissolve a female congregation which was at a certain level, because this slavery of women had entered it - slavery, even to the point of sexual slavery - on the part of clerics or the founder."
Alessandro Gisotti of the Vatican press office later told CBS News that the order in question had been based in France.
Pope Francis said sexual abuse of nuns was an ongoing problem but happened largely in "certain congregations, predominantly new ones".
Last November, the Catholic Church's global organisation for nuns denounced the "culture of silence and secrecy" that prevented them from speaking out.
Just days ago the Vatican's women's magazine, Women Church World, condemned the abuse, saying in some cases nuns were forced to abort priests' children - something Catholicism forbids.
The magazine said the #MeToo movement meant more women were now coming forward with their stories.