Pope Francis has called into question the Christianity of Republican presidential candidate Donald Trump as a result of his views on immigration.
The GOP front-runner has controversially pledged to build a wall to prevent Mexican immigrants entering the US, and to deport nearly 11 million undocumented immigrants.
On a flight from Latin America, Francis told reporters "a person who thinks only about building walls... and not of building bridges, is not Christian. This is not the gospel", he remarked.
When asked whether American Catholics should vote for Trump, Francis replied "I am not going to get involved in that. I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and in this I give the benefit of the doubt," he said.
In a statement, Trump hit back that the Pope Francis was "not Christian" because of his views on immigration.
"If and when the Vatican is attacked by the 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," Trump said in a speech in South Carolina, using an acronym for jihadist group also known as Islamic State.
Trump blamed Mexico for the Pope's remarks, claiming "[The pope] said negative things about me. Because the Mexican government convinced him that Trump is not a good guy".
On the last day of his visit to Mexico, Trump said mass on the US border, and railed against US immigration policies that he claimed drive many into the hands of drugs gangs and human smugglers.
Trump currently holds the lead over rival Republican candidates, of whom Florida Senator Marco Rubio and Jeb Bush are practicing Catholics.
Trump has courted the evangelical Christian vote, and claims to be a Presbyterian Christian, but when asked was unable to cite his favourite passage from the Bible.