Pope Francis attends his weekly general audience in St. Peter's Square at the Vatican. Picture: Riccardo De Luca/AP

Vatican City - Only heterosexual families can form a family, Pope Francis said on Saturday, when he also condemned abortion and hailed the "sanctity" of women who forgive cheating husbands.

"It is painful to say this today: People speak of varied families, of various kinds of family," but "the family [as] man and woman in the image of God is the only one," Francis said in unscripted remarks.

"Many women - but even men sometimes do it [with wives] - wait in silence, looking the other way, waiting for their husband to become faithful again," the pope was quoted as saying by ANSA news agency.

This, he suggested, is "the sanctity that forgives all out of love."

Speaking to the Forum delle Famiglie, an Italian lay movement representing Catholic families, the pope also evoked Nazi eugenics programmes to condemn the abortion of seriously ill foetuses.

"In the last century, the entire world was scandalized by what the Nazis did to ensure the purity of the race. Today we do the same, but with white gloves," he said.

Having criticized the church for focusing too much on abortion and contraception, and saying "who am I to judge?" about homosexuals, the pope is commonly seen as more liberal than his predecessors.

However, the leader of the world's 1.3 billion Catholics has not changed church teaching on family planning and sex - he only comments less frequently, or less exclusively, on such issues.