Pope Francis said on Saturday he would be "sincere" with U.S. President Donald Trump over their sharp differences on subjects such as immigration and climate change when the two hold their first meeting at the Vatican later this month.

But the pope also told reporters aboard a plane returning from Portugal that he would keep an open mind and not pass judgement on Mr Trump until first listening to his views at their meeting on May 24.

"Even if one thinks differently we have to be very sincere about what each one thinks," he said in a typically freewheeling airborne news conference.

"Topics will emerge in our conversations. I will say what I think and he will say what he thinks. But I have never wanted to make a judgement without first listening to the person."

The pope's meeting with Mr Trump could be potentially awkward given their diametrically opposed positions on immigration, refugees and climate change, which he told reporters on the plane "are well known".

Last year, in response to a question about then-candidate Trump's views on immigration and his intention to build a wall along the U.S. border with Mexico, Francis said a man with such views was "not Christian".