Pope Francis has told nuns from around the world that they must be spiritual mothers and not "old maids".

Francis also warned the sisters against using their vocations for personal ambition, saying priests and sisters who do so "do more harm to the church".

He has complained frequently about such "careerism" in the church - a buzzword that is frequently used to describe Holy See bureaucrats.

The Pope made the comments during an audience with about 800 sisters attending an assembly of the International Union of Superiors General.

Leaders of women's religious orders from 75 countries gathered for this morning's event.

The meeting came before Francis' general audience in St Peter's Square, where in a break with tradition, he walked around a quadrant of the square greeting pilgrims.