Pope Francis has branded homosexuality "fashionable" while voicing his concerns about having gay people in the clergy.

The Argentinian pontiff made the remarks in an interview about religious vocations, saying the Church had to be "demanding" in choosing candidates for the priesthood.

Italian newspaper Corriere della Sera's website ran excerpts of the book on Saturday in the form of an interview that Pope Francis gave about religious vocations. Pope Francis was quoted as describing homosexuality within the walls of seminaries, convents and other religious places where clergy live as "a very serious question."

"In our societies, it even seems homosexuality is fashionable. And this mentality, in some way, also influences the life of the church," Pope Francis was quoted as telling his interviewer, a Spanish-born missionary priest, Fernando Prado.

The book, based on four hours of conversations the two had in August at the Vatican, will be published in 10 languages next week. Its Spanish title is La Fuerza de la vocacion (The Strength of Vocation).

Pope Francis reiterated past Vatican pronouncements about the attention that must be given to selecting men for admission to seminaries, saying "we must very much take care of human and sentimental maturity" when training future priests.

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Separately, the Italian news agency ANSA quoted Pope Francis in the book as commenting on a clergyman who had told him having gay people in Catholic religious housing "isn't so grave" because it is "only an expression of affection."

That reasoning "is in error," Pope Francis said. "In consecrated life and priestly life, there is no place for this kind of affection."

He said candidates with "neuroses or strong unbalances" should not be accepted "to the priesthood nor to (other forms of) consecrated life."

Still, Pope Francis, as he has in the past, stressed that gay Catholics contribute to the life of the church. He said the church must always remember "they are persons who will live in the service of the church, of the Christian community, of the people of God. Let's never forget this perspective."

Pope Francis in his papacy has sought to stress that while obeying church teachings, the faithful must also be compassionate and open to others with different views.

Catholic teaching considers homosexual activity sinful, and teaches that everyone, except married heterosexual couples, should abstain from sex.