Pope Francis has intervened explosively in the US election campaign, saying Donald Trump cannot claim to be a Christian while vowing to build a border wall to keep out immigrants.

Key points: Pope says Trump is "not Christian"

Pope says Trump is "not Christian" Comments stem from Trump's views on immigration

Comments stem from Trump's views on immigration Trump says remarks are "disgraceful"

"Anyone, whoever he is, who only wants to build walls and not bridges is not a Christian," the pontiff told journalists during his return journey from a trip to Mexico.

He was responding to a question about the Republican White House hopeful's anti-immigrant stance.

"Vote, don't vote, I won't meddle. But I simply say, if he says these things, this man is not a Christian," Pope Francis said.

Will Trump v Francis affect the polls? The absolute media maelstrom around Donald Trump has only gotten that much bigger as they head into the South Carolina primary, where he's courting the evangelical vote. There's a national poll out saying Senator Ted Cruz has passed him but he's been the frontrunner in the Republican presidential campaign since he began in June last year. He won in New Hampshire, came second in Iowa, and he's heading into South Carolina with a 15-20 per cent lead on some people.

If the polls bear out, he's going to win that, and I don't think Americans are going to be too upset — at least people that would potentially support Donald Trump — that some guy who lives in Rome and is the leader of the Catholic Church has attacked their guy, their man — their bulwark against the elite, so to speak. Analysis by North America correspondent Michael Vincent

"We need to see if he really said them and for this I will give him the benefit of the doubt."

Despite that qualifier, Pope Francis's remarks drew swift and angry condemnation from the billionaire tycoon.

"For a religious leader to question a person's faith is disgraceful," Mr Trump said in a statement delivered on a campaign stop in South Carolina.

"I am proud to be a Christian and as president I will not allow Christianity to be consistently attacked and weakened."

Mr Trump has gained popularity by claiming Mexico is sending criminals to the United States, and last week he accused the Pope of visiting the border between the two countries at the bidding of the Mexican government.

Pope Francis was speaking after concluding a five-day trip to Mexico, where he delivered a mass before 300,000 people near the US border and decried the "human tragedy" of migrants fleeing violence worldwide.

In a highly symbolic gesture the pontiff climbed a ramp facing the Rio Grande and looked out across the border into US territory, where hundreds of migrants waved at him.

Mr Trump has vowed to build a wall on the US southern border to keep migrants from illegally crossing into the United States, a pledge that has caused a firestorm in the presidential campaign, where immigration is a hot-button issue.

His reaction to the Pope's comments was scathing.

"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," Mr Trump said, using an acronym for the militant group.

'Pope used as pawn'

Propagandists for the Islamic State group have issued threats that their fighters will plant their flag on the top of St Peter's basilica.

But Italian officials have insisted they have no knowledge of any credible threat to the Vatican or the Pope.

Mr Trump said Mexican authorities had told the Pope only "one side of the story".

"He didn't see the crime, the drug trafficking and the negative economic impact the current policies have on the United States," 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."

It was not the first time Mr Trump had issued the pawn jibe and Pope Francis was asked about it on the plane.

"Am I a pawn of the Mexican government? I leave that to your judgement, to the people to judge," he said.

The 79-year-old Argentinian, a fervent critic of the freewheeling capitalism espoused by Mr Trump, also said he was proud to have been branded a politicised pontiff by the tycoon.

"Thanks be to God if that is what he said, because Aristotle defined man as a political animal. At least [it means] I am a human person."

Mr Trump has promised to end illegal immigration by building a wall along the Mexican border.

His first TV ad, unveiled on Monday, uses footage that shows migrants fleeing Morocco to the tiny Spanish enclave of Melilla in 2014.

The Mexican-US border stretches more than 3,100 kilometres, but only around a third of it is currently covered by high-security fencing.

Bush, Rubio back a secure border

Mr Trump was backed by the head of the world's largest Christian university Jerry Falwell Jr, who in response to the spat went on CNN and appeared to liken Mr Trump to Jesus Christ.

"Donald Trump does not wear his religion on his sleeve, but I believe he's a man of faith," he said.

"I can only quote Jesus Christ who said 'judge not less ye be judged'.

"Jesus called the religious elite of his day, hypocrites, a generation of vipers and he called them wolves in sheep's clothing.

"So if what Donald Trump said was bad then what Jesus said is bad because Jesus questioned religious leaders who judged others and that's exactly what Donald Trump said in his comments today."

Republican presidential candidates and Catholics, senator Marco Rubio and Jeb Bush, also responded, saying it was their jobs to make the border secure.

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"I haven't seen his statement, but I'm an elected official in the United States of America ... and our number one obligation is to make people safe and our immigration system is a part of that," Mr Rubio said.

Mr Bush said: "That's not an un-Christian thing to do, to make sure that people don't come across our border illegally.

"That's a just thing to do."

US media reminded Mr Trump about his response to the Pope in 2013 when he tweeted: "The new Pope is a humble man, much like me, which is probably why I like him so much..."

AFP/ABC