The Pope has questioned US Republican presidential candidate Donald Trump's Christianity.

On a flight back to the Vatican from a visit to Mexico, Pope Francis was asked about Trump and other candidates' proposals to build a wall along the US border with Mexico to keep out migrants.

The Pope told reporters: "A person who only thinks about building walls, wherever they may be, and not building bridges, is not Christian."

Trump responded angrily to the comments, saying in a statement on his website the Pope would have "wished and prayed" Trump was president "If and when the Vatican is attacked by Islamic State."

He said: "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".

He went on to add that it was "disgraceful" for a religious leader to question a person's faith.

"For a religious leader to question a person's faith is disgraceful. I am proud to be a Christian and as President I will not allow Christianity to be consistently attacked and weakened, unlike what is happening now, with our current President", Trump said.

And he went on to accuse the Mexicans of using the Pope as "a pawn".

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