Pope Leo initially impressed conservative Catholics after his election in May as he embraced traditions shunned by his predecessor Pope Francis and steered clear of hot button social issues that divided the 1.4 billion-member Church.

But his honeymoon with conservatives appears over after he unexpectedly took aim at U.S. President Donald Trump's hard-line immigration policies, questioning whether they were in line with the 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," Leo, the first U.S. pope, told reporters on Tuesday.

Some critics, who had praised the pope for his early reserve, expressed shock that Leo criticized the current champion of global conservatives.

Former Texas Bishop Joseph Strickland, a fierce Francis critic who was relieved from duty by the late pope but has praised Leo, criticized the new pope on social media for causing "much confusion … regarding the sanctity of human life and the moral clarity of the Church's teaching."

"So tired of papal interviews. He should return to his previous silence," opined the Rorate Caeli blog, which had previously criticised Francis and praised Leo.

The Trump administration, which was sharply critical of Francis but has rarely commented about Leo, also pushed back.

Press secretary Karoline Leavitt said she rejected the characterisation of inhumane treatment of immigrants.