The Catholic Church must show "zero tolerance" to sexual assault by members of the clergy, Pope Francis said in excerpts of an interview with a Portuguese television channel broadcast Sunday.

"It's very clear. It's zero tolerance," he told TVI/CNN Portugal in an interview, extracts of which were published on the channel's website.

"A priest cannot continue being a priest if he is an aggressor. He cannot because he is either sick, or a criminal," he said.

"It's monstrous because it destroys lives," he added, during the two-part interview to be aired on Sunday and Monday.

An independent inquiry into sexual abuse within the Catholic Church in Portugal has so far gathered testimony from around 400 people, said the man leading the inquiry, child psychiatrist Pedro Stretch.

As a result, 17 cases have been referred to the judiciary. The findings of the inquiry are expected at the end of the year.

"I do not deny abuses," said the pope. "A single abuse would already be monstruous."

He said he hoped to visit Portugal in August next year for World Youth Day, a gathering of young Catholics.

"I think I will go. In any case, the pope will go," he said.

The pontiff has raised the possibility of retiring due to his declining health.