Pope Francis has acknowledged nuns were sexually abused by clergy members for the first time.

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.

"It's true, it's a problem" he told reporters on the papal plane from Abu Dhabi to Rome.

"The mistreatment of women is a problem. I would dare to say that humanity still hasn't matured."

"Let's begin here: it's a cultural problem".

"It's true, within the Church there have been clerics who have done this.

"In some civilisations a little stronger than in others. It is not a thing that all have done.

"There have been priests and also bishops who have done that. And I believe that it may still be being done."

Pope Francis talks with journalists aboard an airplane on his way to the United Arab Emirates | Image: Andrew Medichini/AP/Press Association Images

"It's not a thing that from the moment in which you realize it, it's over. The thing goes forward like this.

"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."

He added: "Pope Benedict had the courage to dissolve a women's congregation that had a certain level because this slavery of women had entered, even sexual slavery, by clerics or by the founder.

"Sometimes the founder takes the freedom, empties the freedom of the sisters, it can arrive to this."

Alessandro Gisotti, interim director of the Vatican press office, told CBS News a female congregation based in France was dissolved in 2005 under Pope Benedict due to the extent of abuse in it.

The issue was brought to the fore after the February issue of monthly magazine Women Church World, 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.