Pope Francis has criticised Republican presidential candidate Donald Trump's views on immigration, saying they are "not Christian".

Speaking to reporters on a return flight to Rome from Mexico, the pontiff said the GOP frontrunner's campaign promise to build a wall along the US Southern border did not follow the gospel.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said when asked specifically about Mr Trump's immigration platform.

"This is not in the gospel."

However, the Pope stopped short when asked if he would advise US Catholics not to vote for the billionaire.

"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."

Mr Trump responded by saying Pope Francis will have wanted him in the White House "if and when" Islamic State militants attack 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," he said in a speech in South Carolina.

The Presbyterian businessman added: "No leader, especially a religious leader, should have the right to question another man's religion or faith."

He also accused Mexico's leaders of "using the Pope as a pawn".

Mr Trump has promised throughout his campaign to build a wall along the US-Mexico border and to deport some 11 million immigrants living illegally in the US.

He has also proposed a temporary ban on Muslims entering the United States.

The pontiff capped his five-day trip to Mexico by praying at the country's border for people who died trying to reach the United States.

His comments on Thursday came amid a freewheeling conversation with reporters, during which he also addressed a potential softening of the Church's ban on contraception because of the Zika virus.