The Pope, who was travelling back to Rome from Mexico, where he urged the US to address the "humanitarian crisis" on its southern border, did not tell American Catholics not to vote for Trump, media reported.

But Francis left little doubt where he stood on the polarizing issue of immigration reform."A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not the gospel," the Pope told journalists who asked his opinion on Trump's proposals to halt illegal immigration.

Trump immediately fired back, calling Francis' comments "disgraceful"."No leader, especially a religious leader, should have the right to question another man's religion or faith," he said in statement.

Trump added that the government in Mexico, where Francis spent the past five days, has "made many disparaging remarks about me to the Pope".

The tussle between Trump and Francis - two outsized personalities who seldom shy from speaking their minds - seems to have been building for some time. Before the Pope travelled to Mexico, Trump cast the pontiff as a political naif who "doesn't understand the dangers" at the US-Mexican border.

The Pope appeared somewhat unaware of Trump's exact stance on illegal immigration, though, saying that he would give him "the benefit of the doubt" until he had heard exactly what the billionaire businessman had said.

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