Pope Francis told 800 nuns gathered at the Vatican on Wednesday that they should be spiritual "mothers" rather than "spinsters", and stressed the importance of showing obedience to the Catholic Church.

The pontiff, 76, told the assembly of the International Union of Superiors General they should strive for "a fertile chastity, a chastity which produces spiritual children within the Church."

"The ordained woman is a mother, she must be a mother and not a spinster! You are mothers, like the figures of Mary and the Mother Church," he told the nuns representing women's religious orders from some 75 countries.

"It is not possible to understand Mary without maternity, nor the Church without motherhood," he added.

The Argentine pontiff, who in April backed a report drawn up under his predecessor Benedict XVI accusing nuns in the United States of rebellion, said "obedience is to listen to the will of God."

"Remember that the relationship between authority and obedience is located within the context of the mystery of the Church," he said.

Francis reaffirmed the findings of the report which said that the Leadership Conference of Women Religious (LCWR) -- which represents around 45,000 US nuns and is known for its social work -- held "radical feminist" views.

While the Vatican acknowledged the group's work in schools, hospitals and poor houses, it also said it should cooperate with US bishops and said LCWR was legally "under the direction of the Holy See".