Pope Francis has suggested women threatened with the Zika virus could use artificial contraception, saying "avoiding pregnancy is not an absolute evil" in light of the global epidemic.
The pontiff unequivocally rejected abortion as a response to the crisis in remarks on Wednesday as he flew home after a five-day trip to Mexico.
But he drew a parallel to a decision by Pope Paul VI in the 1960s to approve giving nuns in Belgian Congo artificial contraception to prevent pregnancies because they were being systematically raped.
Abortion "is an evil in and of itself, but it is not a religious evil at its root, no? It's a human evil," Francis told reporters.
"On the other hand, avoiding pregnancy is not an absolute evil. In certain cases, as in this one (Zika), such as the one I mentioned of Blessed Paul VI, it was clear."
Francis was responding to a reporter's question about whether abortion or birth control could be considered a "lesser evil" when confronting the Zika crisis in Brazil, where there has been a spike in babies born with abnormally small heads to Zika-infected mothers.
The World Health Organisation has declared a worldwide health emergency over the Zika virus and its suspected link to birth defects.
On Thursday, the UN agency advised the sexual partners of pregnant women to use condoms or abstain from sex if they live in or have visited Zika-affected areas, echoing a recommendation made by US health officials.
The virus has been reported in at least 34 countries, most of them in overwhelmingly Catholic Latin America and the Caribbean, where access to birth control is often limited and abortion is heavily restricted.
Theologians and some Latin American bishops cautioned the Pope was not giving a green light for Catholics to use artificial birth control, nor did his remarks amount to a change in church teaching.
But Francis' comments suggest Catholics under specific circumstances could make a "conscience-based decision about whether they should prevent pregnancy", said the Reverent James Bretzke, a moral theologian at Boston College.
"Certainly Pope Francis should have anticipated a question on the church's response to the Zika virus, and from his answer I believe he was well prepared to field this query," he said.
"His comment on the Zika virus and measures to avoid pregnancy is within perfect consistency with the traditional moral teaching of the Catholic Church."
Several Latin American churchmen have reasserted the church's opposition to abortion and artificial contraception as more reports of Zika cases and brain-damaged babies emerged.
On Thursday, Bishop Noel Antonio Londono of the Diocese of Jerico in the Colombian state of Antioquia, said nobody should interpret the Pope's remarks as licence to use birth control.
"This isn't a blank cheque," he said. "Undoubtedly, with regards to Zika, there are people who need to protect themselves."
