Pope Francis has appeared to open the door to a possible limited softening of the Roman Catholic Church's ban on contraception because of the Zika virus.
But the Argentine pontiff, speaking to reporters as he flew back to Rome from a visit to Mexico, categorically ruled out abortion as a response to Zika, comparing the practice to a Mafia killing.
The health crisis has put pressure on Church teachings, particularly in Latin America, where abortion is now being debated more openly even in some conservative countries.
Many scientists believe Zika, a mosquito-borne disease that is currently sweeping through the Americas, may be a risk factor for microcephaly in newborns - a condition in which babies are born with abnormally small heads.
In the freewheeling, post-trip news conference that has become a trademark of his papacy, the pope was asked if using contraception would fall into the category of the lesser of two evils and how he felt about some authorities advising pregnant women with Zika to have abortions.
He categorically ruled out that abortion would ever be permitted for pregnant women with Zika who fear that they may give birth to a child with microcephaley.
"Abortion is not a lesser evil. It is a crime. It is killing one person to save another. It is what the Mafia does," Francis said, speaking passionately against the practice. "It is a crime. It is an absolute evil."
The 1.2 billion-member Church teaches that abortion is a crime because life begins at the moment of conception. The Church teaches that contraception is wrong because nothing should block the possible transmission of life.
But Francis mentioned in his response that one of his predecessors, Pope Paul VI, had issued an exceptional dispensation allowing nuns in Africa to use the birth control pill because they risked being raped during a conflict there.
He said Paul, who reigned from 1963 to 1978, had responded to "a difficult situation in Africa", suggesting that a papal precedent existed.
Francis did not say exactly when his predecessor made the exception, but it was believed to be in the 1960s in what was then the Belgian Congo. Little is known of the episode, which was not publicised at the time.
Francis said that unlike abortion, "avoiding pregnancy is not an absolute evil" and added that "in certain cases", such as the precedent set by Paul VI regarding the nuns in Africa, using contraception might be the "lesser evil".
He did not elaborate.
Paul VI confirmed the Roman Catholic ban on artificial birth control in his major 1968 encyclical "Humanae Vitae" (On Human Life), which ran into strong opposition in some parts of the Church.
In his comments on Zika, Francis also urged the medical and scientific communities to do every possible to find out more about the disease.
"I would also like to exhort doctors to do everything to find vaccines against the mosquitoes that bear this illness. We have to work on that," he said.