Thrusting himself into the heated American presidential campaign, Pope Francis declared Thursday that Donald Trump is "not Christian" if he wants to address illegal immigration only by building a wall along the U.S.-Mexico border.

Trump fired back ferociously, saying it was "disgraceful" for a religious leader to question a person's faith.

Trump, a leading U.S. Republican presidential candidate, has promised to build a wall along the Mexican border from Texas to California and expel 11 million people who are in the country illegally if elected president.

The Pope's comments en route home from Mexico came hours after he prayed at the Mexico-U.S. border for people who died trying to reach the United States.

U.S. Republican presidential candidate Donald Trump greets members of the Sun City Republicans on Thursday after speaking at their gated retirement community in Bluffton, South Carolina. (Jonathan Ernst/Reuters)

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said. "This is not in the Gospel."

Not having heard Trump's border plans independently, Francis said he'd "give him the benefit of the doubt." But he added: "I'd just say that this man is not Christian if he said it this way."

Texas Senator Ted Cruz, another Republican presidential contender, has also supported building a border wall, and joked that he will make Trump pay for it.

Trump, a Presbyterian, last week criticized Francis's plans to pray at the border. He said the move was ill-informed and showed Francis to be a political figure being exploited by the Mexican government.

"I don't think he understands the danger of the open border that we have with Mexico," Trump said in an interview with Fox News. "I think Mexico got him to do it because they want to keep the border just the way it is. They're making a fortune, and we're losing."

On Thursday, he responded to the Pope's comments during a campaign stop in Kiawah Island, S.C.

"No leader, especially a religious leader, should have the right to question another man's religion or faith," he said. "They are using the Pope as a pawn and they should be ashamed of themselves for doing so, especially when so many lives are involved and when illegal immigration is so rampant."

He also said the Mexican government has disparaged him to the Pope and separately invoked the Islamic State group, saying that if it attacks the Vatican, "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."

Asked if he felt he was being used as a pawn of Mexico, Francis said he didn't know.

"I leave that judgment to you, the people."

But he seemed quite pleased to hear that Trump had called him a "political" figure, noting that Aristotle had described the human being as a "political animal."

Zika crisis

Francis also suggested that women threatened with the Zika virus could use artificial contraception, saying there's a clear moral difference between aborting a fetus and preventing a pregnancy.

Francis was asked if abortion or birth control could be considered a "lesser evil," when confronting the Zika crisis in Brazil, where some babies have been born with abnormally small heads to Zika-infected mothers.

The explosion of Zika cases has prompted some governments in Latin America to urge women to avoid getting pregnant and has fueled calls from abortion rights groups to loosen the strict anti-abortion laws in the overwhelmingly Catholic region.

But Francis excluded abortion absolutely from the debate.

"Abortion isn't a lesser evil, it's a crime," he told reporters. "Taking one life to save another, that's what the Mafia does. It's a crime. It's an absolute evil."

Francis, however, drew a parallel to the decision taken 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," he said. "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."