Republication presidential candidate Donald Trump is "not Christian" due to his views on immigration, Pope Francis said on his way back to Rome from Mexico Thursday. However, the pope said he did not want to advise American Catholics about voting related to Trump.

Francis dug into Trump's vow to build a wall between the United States and Mexico if he becomes president, during an hour-long, freewheeling conversation with reporters on the papal flight, reports Religion News Service. He touched on other hot-button topics, too, including pedophilia, the spread of the Zika virus and his meeting with Patriarch Kirill.

Trump struck back, swiftly dismissing the leader of the world's Roman Catholics as "disgraceful" for questioning his faith, reports Reuters.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said in answer to a specific question about Trump's views. "This is not in the gospel."

Asked if American Catholics should vote for someone with Trump's views, Francis said:  "I am not going to get involved in that. 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 in this, I give the benefit of the doubt."

On his last day in Mexico, Francis said a Mass on the U.S. border, where he spoke against immigration policies that force many people underground and into the hands of drug gangs and human smugglers.

Trump also said he would deport 11 millions of illegal migrants if he wins the November U.S. election. Last week, he told Fox Business television that Pope Francis did not understand the Mexican border issues.

"The pope is a very political person. I think he doesn't understand the problems our country has. I don't think he understands the danger of the open border that we have with Mexico," Trump said, who also reportedly referenced the pope as a pawn, an instrument of the Mexican government for migration politics.

Asked about being called a "political person," Francis said: "Thank God he said I was a politician because Aristotle defined the human person as 'animal politicus.' So at least I am a human person."

The pope answered 12 questions posed in English, Spanish and Italian while on this overnight flight.