He said the church was aware of priests and bishops abusing nuns
Pope Francis has said nuns were sexually abused by clergy members.
He said in one case, they were kept as 'sex slaves' and admitted the problem "may still be being done".
The pontiff insisted the Catholic Church was aware of priests and bishops abusing nuns.
He told reporters on the papal plane from Abu Dhabi to Rome: "It's true, within the Church there have been clerics who have done this".
"There have been priests and also bishops who have done that. And I believe that it may still be being done."
Must something more be done? Yes.
"We've been working on this for a long time. We've suspended some clerics, sent them away for this, and also - I don't know if the process is finished - dissolved some women's religious congregations that were very tied up in this, a corruption.
"Must something more be done? Yes. Do we have the will? Yes."
A female congregation based in France was dissolved in 2005, under Pope Benedict, because of the extent of abuse.
The issue was brought to light after the February issue of a monthly woman's magazine, which is distributed alongside Vatican newspaper L'Osservatore Romano, was published.
It said nuns have been silenced for years by fear of retaliation against them, or their orders, if they reported the priests who abused them.