VATICAN CITY (AP) — Pope Francis vowed Friday to combat the sexual abuse of nuns and urged religious sisters to just say no when clergy want to use them as maids.

Francis told 850 superiors of religious orders gathered for the triennial assembly of the International Union of Superiors General, the main umbrella group of nuns, that theirs is a vocation of service, not servitude.

The union's president, Maltese Sister Carmen Sammut, told Francis that clergy abuse of sisters was "diffuse in many parts of the world," and included sexual abuse, spiritual abuse, as well as taking of their property. She added that there were also cases of nuns abusing other nuns.

Francis said sexual abuse of sisters was "a serious, grave problem" of which he was well aware.

"We have to fight this, and also the service of religious sisters: Please: Service yes, servitude no."

Francis noted that sometimes the problem lies with the superior herself, a reference to cases where religious superiors submitted to demands placed on them by bishops, to the detriment of her own sisters and novices.

"You didn't become a sister to become a priest's maid," Francis told the sisters in a Vatican audience hall. He said if they want to do domestic work, they should join orders that care for the elderly in old-age homes because "that is service, not servitude."

Francis was also asked about his recent comments about the role of female deacons in the early Christian church. Francis said this week that scholars on a Vatican commission looking into the role of women deacons failed to come to a consensus on whether they received the same sacramental ordination as men.

The pope said the issue required further study "because I can't make a sacramental decree without a theological or historical foundation."

Francis was then pressed by a German nun, who said the church must not rely solely on historic precedent for a future decision about female deacons, but on the needs of the church today.

Francis said that was true, but said he can't do something that hasn't developed from Revelation, the truths about the faith revealed by God.

"If I see that this has a connection with Revelation, OK. But if it's something strange, that isn't in Revelation or the moral field, it's not OK," Francis said. "If the Lord didn't want a sacramental ministry for women, it can't go forward."

Deacons today are ordained ministers, not priests, though they can perform many of the same functions as priests. They preside at weddings, baptisms and funerals, and they can preach. They cannot celebrate Mass.

Currently, married men can serve as deacons. Women cannot.

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