Pope Leo on Tuesday appeared to offer his strongest criticism yet of U.S. President Donald Trump's hard-line immigration policies, questioning whether they were in line with the Catholic Church's pro-life teachings.

"Someone who says I am against abortion but I am in agreement with the inhuman treatment of immigrants in the United States, I don't know if that's pro-life," the pontiff told journalists outside his residence in Castel Gandolfo.

The Catholic Church's position that life is sacred from conception until natural death is one of the 1.4-billion-member denomination's strongest teachings.

Leo, the first U.S. pope, was responding to a question from a U.S. journalist who asked about the country's politics.

The White House said Trump was elected based on his many promises, including to deport criminal illegal aliens. "He is keeping his promise to the American people," spokesperson Abigail Jackson responded in a statement.

Elected in May to replace the late Pope Francis, Leo has shown a much more reserved style than his predecessor, who frequently criticised the Trump administration.

Leo was asked about a decision by the Archdiocese of Chicago to give an award to Illinois Senator Dick Durbin, a Democrat who supports abortion rights. The move has attracted vocal criticism from conservative Catholics, including several U.S. bishops.

"It is very important to look at the overall work that the Senator has done," said the pope.

"I understand the difficulty and the tensions, but I think, as I myself have spoken in the past, it's important to look at many issues that are related to what is the teaching of the Church," he said.

"Someone who says I am against abortion but says I am in favour of the death penalty is not really pro-life," said the pope.