U.S. Republican presidential candidate Donald Trump holds a rally with supporters in Gaffney, South Carolina February 18, 2016.

Pope Francis said Donald Trump is "not a Christian", criticising the Republican presidential candidate for his demand to deport millions of Mexicans illegally staying in the United States.

"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," the Pope said during his six-day visit to Mexico.

"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 added.

Irascible Trump has called for the building of a border wall with Mexico that will prevent migrants streaming into the country. He had earlier said Mexico was sending criminals and rapists to the US illegally.

Trump, who called for the deportation of 11 million undocumented migrants, responded to the Pope's comments saying he is a "proud Christian'. "For a religious leader to question a person's faith is disgraceful. I am proud to be a Christian," Trump said.

He also blamed Mexico for the Pope's comments. "The pope said negative things about me. Because the Mexican government convinced him that Trump is not a good guy," he said.

The controversial Republican also mocked the Pope saying when the Islamic State attacks the Vatican the pontiff "would have only wished and prayed that Donald Trump would have been president."

Trump's Republican rivals Marco Rubio and Jeb Bush, who are Catholics, tried to stay away from the duel, saying they look at the pontiff for spiritual guidance, not political direction.