Pope Francis has admitted that clerics have sexually abused nuns, and in one case kept them as sex slaves, the first time such issues have been acknowledged.

He made the comments on Tuesday while on a historic tour of the Middle East, saying the issue was being addressed, but the problem was ongoing.

Days before, the Vatican’s women’s magazine, Women Church World, condemned the abuse, saying in some cases, nuns were forced to abort priests’ children, something that is forbidden in Catholicism.