Pope Francis has called into question Donald Trump's faith, saying "a person who thinks only of building walls... rather than building bridges, is not a Christian".

The Pope was asked to comment on Trump's threat to build a wall along the southern border of the US with Mexico.

Speaking aboard the papal plane on his trip back to Rome from Mexico today, Francis said: "A person who thinks only of building walls, wherever they may be, and not building bridges, is not Christian. This is not in the gospel".

He refused to comment on whether Americans should vote for Trump in the upcoming elections, but added: "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 I will give him the benefit of the doubt".

Trump has taken a hard line on immigration, pledging to build a wall to slow down the number of Mexicans entering the United States, and even promising to make Mexico pay for it. In July last year, he claimed that Mexican immigrants were responsible for a significant number of rapes in America.

"If you look at the statistics of people coming, you look at the statistics on rape, on crime, on everything coming in illegally into this country it's mind-boggling!" he said in an interview with CNN.

In his speech announcing his bid for presidency, Trump also said: "When Mexico sends its people, they're not sending their best... They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists. And some, I assume, are good people."

The Pope, meanwhile, has been vocal in his condemnation of the harsh treatment of migrants, and yesterday railed against immigration policies during a Mass in Ciudad Juarez, on the Mexico-US border.

"We cannot deny the humanitarian crisis," Francis said.

"Each step, a journey laden with grave injustices: the enslaved, the imprisoned and extorted; so many of these brothers and sisters of ours are the consequence of trafficking in human beings."

Last week Trump said the Pope was a "very political person", and accused Francis of being an instrument of the Mexican government.

Vatican spokesman Father Federico Lombardi said the allegation was "very strange indeed".

"The Pope always speaks of the problems of immigration. If Mr Trump were to come to Europe he would see that the Pope has said the same things about immigration to the Italians, the Germans, the French and the Hungarians," Lombardi added.