The Pope has questioned US Republican presidential candidate Donald Trump's Christianity over his call to build a border wall with Mexico. Photo: BBC

Mr Trump has been courting the evangelical Christian vote in the US. Photo: AP

Pope Francis said "a person who thinks only about building walls... and not of building bridges, is not Christian".

The New York businessman supports deporting nearly 11 million undocumented immigrants.

Calling himself a "proud Christian", Mr Trump blamed Mexico for the Pope's remarks, calling them "disgraceful".

Mr Trump has alleged that Mexico sends "rapists" and criminals to the US.

Pope Francis made the comments at the end of trip to Mexico.

"A person who thinks only about building walls, wherever they may be, and not of building bridges, is not Christian. This is not the gospel," he said.

He declined to say whether Americans should vote for Mr Trump, who is leading the Republican race for president.

"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," the Pope said.

Addressing a rally in South Carolina, Mr Trump responded to the Pope's comments.

"For a religious leader to question a person's faith is disgraceful. I am proud to be a Christian," Mr Trump said. "No leader, especially a religious leader, should have the right to question another man's religion or faith."

"[The pope] said negative things about me. Because the Mexican government convinced him that Trump is not a good guy," he said.

He also said the Vatican is the so-called Islamic State's "ultimate trophy", and "if and when" the group attacks it "the Pope would have only wished and prayed that Donald Trump would have been President because this would not have happened."

Earlier this month, Mr Trump called Pope Francis "a very political person" in an interview with Fox News.

"I don't think he understands the danger of the open border we have with Mexico," Mr Trump said.

"And I think Mexico got him to do it because Mexico wants to keep the border just the way it is, because they're making a fortune and we're losing."

American Catholics are seen as an important voting bloc in US elections. Many support Republican candidates because of their opposition to abortion and gay marriage.

Rival Republican candidates Florida Senator Marco Rubio and Jeb Bush are practicing Catholics.

Mr Trump has been courting the evangelical Christian vote, often successfully, but his fellow Republican rivals have tried to argue his religiosity is not sincere.

Ted Cruz's campaign is now running an advertisement featuring an old television interview Mr Trump gave in which he said he was "very pro-choice" when it comes to abortion.

In January Mr Trump faced ridicule after flubbing a Bible verse when giving a speech to a Christian university in Virginia.

He has said he is a Presbyterian Christian but has had trouble recalling his favourite Bible verse when asked.

He has referred to communion, the Christian sacrament meant to signify Jesus' last supper, as having "the little wine" and "the little cracker."