Pope Francis called on religious sisters Tuesday to "fight" when treated unfairly, acknowledging that some are "reduced to servitude" working for Catholic clergy.

The 85-year-old dedicated his regular monthly prayer video to religious sisters and consecrated laywomen, thanking them for their work.

"I invite them to fight when, in some cases, they are treated unfairly, even within the Church, when they serve so much that they are reduced to servitude - at times, by men of the Church," the pontiff said in a video.

"Do not be discouraged. May you keep making God's goodness known through the apostolic works you do. But above all through your witness of consecration.

"Let us pray for religious sisters and consecrated women, thanking them for their mission and their courage. May they continue to find new responses to the challenges of our times."

The exploitative treatment of nuns assigned to household chores in the service of the male hierarchy of the Catholic Church, notably cardinals, was highlighted in a 2018 report by the Vatican daily, Osservatore Romano.

"They are women who work as servants" for priests, bishops and cardinals, "cooking, cleaning, washing clothes" and they are "very badly paid", said Lucetta Scaraffia, the journalist who led the investigation published in a women's supplement of the paper.

"It's considered normal in the Church - women must serve without asking for anything in return," Scaraffia, author of a book "Feminist and Christian", told AFP.