Pope Francis has said that anyone who wants to build a wall along the US-Mexican border is not Christian, when questioned about US presidential candidate Donald Trump's views on immigration.
Mr Trump, a leading 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.
In response, Mr Trump claimed a religious leader questioning a person's faith is "disgraceful" and the leader of the Roman Catholic Church "only heard one side of the story".
The Pope's comments on his way home from Mexico came hours after he prayed at the Mexico-US border for people who died trying to reach the US.
"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 Mr Trump's border plans independently, Francis said he would "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."
Mr Trump, during a campaign stop in Kiawah Island, South Carolina, responded by saying: "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.
"No leader, especially a religious leader, should have the right to question another man's religion or faith."
Texas Senator Ted Cruz, another Republican presidential contender, has also supported building a border wall and joked he will make Mr Trump pay for it.
Mr Trump, a Presbyterian, last week criticised 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," Mr 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."
Asked if he felt he was being used as a pawn of Mexico, Francis said he did not know. "I leave that judgment to you, the people."