Pope Francis tries on a cap he was offered as he leaves at the end of his weekly general audience, in St. Peter's square, at the Vatican. Picture: Andrew Medichini/AP

Rome - Homosexuality in the Catholic clergy is a continuing source of concern for Pope Francis, according to excerpts from a Spanish-language interview set to be published in a new book on Monday.

"In our society, it even seems that homosexuality is a trend, and this mentality is influencing our Church in certain ways," he said.

The fact that there are homosexuals in the clergy and seminars is a source of concern. He called is a "very serious matter."

People with "these deep-seated tendencies" should not be allowed into seminars, in his opinion.

The interview appears in a book by Fernando Prado. Titled "La Fuerza de la Vocacion" ("The Strength of Vocation"), it is set for release in multiple languages. Most of the book focuses on Francis discussing his calling to religious service.

The Catholic Church is opposed to people engaging in homosexual behaviour, but Francis has, in the past, spoken out against discrimination against homosexuals.