Pope Francis is "concerned" about what he described as the "serious issue" of homosexuality, saying in an interview published Saturday that being gay was a "fashion" to which the clergy is susceptible.

"The issue of homosexuality is a very serious issue that must be adequately discerned from the beginning with the candidates," the pontiff said with regards to would-be priests.

"In our societies it even seems that homosexuality is fashionable and that mentality, in some way, also influences the life of the church," he says in the book 'The Strength of a Vocation', released in Italy on Saturday.

"This is something I am concerned about, because perhaps at one time it did not receive much attention," he said in the book, a transcript of an interview which will be released in ten languages.

The Roman Catholic Church's position is that homosexual acts are sinful and the pontiff's stance on homosexuality in the clergy is not new.