The admission by the pontiff marks the first public acknowledgement of the problem, which piles further scandal onto Catholic clergy who are already coping with a crisis of global proportions over the sexual abuse of children.
In recent years, Catholic nuns have raised their voices over sexual assaults by clerics in Africa, Latin America, India, and Italy.
Speaking to an Associated Press reporter on Tuesday during a flight home from Abu Dhabi, the pope acknowledged that more must be done to address the crisis:
"It's true … There have been priests and even bishops who have done this. I think it is still going on because something does not stop just because you have become aware of it."
The pope also revealed that an order of nuns was dissolved by the previous pontiff, Pope Benedict, after abuses "to the point of sexual slavery" were committed by high-ranking clergy. Pope Francis said:
"It's a path that we've been on. 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."