The leader of the Catholic Church was flying home from his visit to Mexico on Thursday when a reporter asked him if birth control and/or abortion could be considered lesser evils when faced with a birth defect widely believed to be caused by the Zika virus.

The Vatican opposes both contraception and abortion. However, Pope Francis said there was a clear moral distinction between the two.

"Abortion is not a lesser evil," the pope said. "It is a crime. It is killing one person to save another. It is what the Mafia does."

"Avoiding a pregnancy is not an absolute evil," the pope said.

The Zika virus is primarily transmitted by infected mosquitoes, but there have also been cases of sexual transmission. Citing its refusal to bend on the issue of condoms, critics have accused the Vatican of hindering necessary health measures in South American countries, which have majority Catholic populations.

The pope said one of his predecessors, Paul VI, had allowed nuns to use birth control during a humanitarian mission in Africa. According to Francis, the nuns took birth control pills because they were facing risk of rape during a war. Paul VI led the Catholic Church between 1963 and 1978.

Pope Francis did not elaborate on the episode or say when it took place. Some observers, however, suggested that it was most likely during a war in what was then the Belgian Congo.

'Not in the Gospel'

The pope also discussed the proposal by the US billionaire and Republican presidential candidate Donald Trump to deport the 11 million people he says are in the United States illegally and reduce further immigration by erecting a wall on the border with Mexico.

"Anyone, whoever he is, who only wants to build walls and not bridges is not a Christian," Francis said. "This is not in the Gospel."

The pontiff added that he did not listen to Trump's idea himself and that he would give the real estate mogul the "benefit of the doubt."

"I'd just say that this man is not Christian if he said it this way," the pope said.

Pope's criticism 'disgraceful'

Trump, who says he is Presbyterian, was quick to lash out at the pope, declaring himself a proud Christian who would not "allow Christianity to be consistently attacked and weakened."

"For a religious leader to question a person's faith is disgraceful," Trump said in South Carolina.

"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 said, using an alternative acronym for the "Islamic State."

Trump also accused Mexican authorities of telling the pope only "one side of the story."

"They are using the pope as a pawn," Trump said, "and they should be ashamed of themselves for doing so."

dj/cmk (AFP, AP, Reuters, dpa)