In wide-ranging comments aboard the papal plane, Pope Francis suggested to reporters that it might be acceptable for those fearing the Zika virus to use contraception.

The pope did not explicitly approve the use of contraception as he spoke during the flight from Mexico to Rome. But he drew a distinction between the use of abortion to respond to the threat of Zika — which he categorically opposed — and the hypothetical use of contraception.

There are concerns that the Zika virus, currently raging across Latin America, may be linked to cases of microcephaly, a severe birth defect.

A reporter on the plane had asked the pope how he felt about advice from some authorities that women at risk of Zika have abortions, and whether contraception would be the lesser of two evils, The Associated Press reports.

"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, according to an AP translation. "It is a crime. It is an absolute evil."

But contraception is different, the pope said, noting that "avoiding pregnancy is not an absolute evil," the AP writes.

The Vatican press office described the pope's remarks on contraception: "Using contraceptives to avoid pregnancy can be acceptable in difficult situations, he said, noting that Pope Paul VI authorized nuns in Africa to do the same half a century ago when they were threatened with rape."

That exceptional dispensation from Paul VI was not publicized at the time, the AP writes, and little is known about it.

In his remarks, Pope Francis also said that while he wasn't personally familiar with presidential candidate Donald Trump's plans for a wall between the U.S. and Mexico, anyone who proposed such a wall "is not Christian."

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not in the Gospel," the pope said, according to the Catholic News Agency.

Trump has responded, saying in a statement, "If and when the Vatican is attacked by ISIS, which as everyone knows is ISIS's ultimate trophy, I can promise you that the Pope would have only wished and prayed that Donald Trump would have been President because this would not have happened."

He also called it "disgraceful" for the pope to question his faith.

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