Pope Francis warns Trump about border wall

"Builders of walls, be they made of razor wire or bricks, will end up becoming prisoners of the walls they build," the Pope told reporters aboard the Papal plane on Sunday in response to a question about Trump's threat to close the US-Mexico border.

The Pope was flying back to Rome from Morocco, where he had spoken about the plight of migrants during his two-day visit to the Muslim-majority African nation. He also sought to establish closer relations between the Roman Catholic Church and moderate Islam, the New York Times reported.

Trump had threatened on Friday to seal off the border with Mexico and moved to cut aid to Guatemala, Honduras and El Salvador to punish them for failing to stop a surge in migrant crossings into the US.

The Pope has long been a vocal critic of Trump's immigration policies. "A person who thinks only about building walls, wherever they may be and not building bridges, is not Christian," he said in 2016, referring to the US President's plans to prevent migrants from entering the US at the southern border.

The pontiff also criticised Trump in 2018 for his administration's policy of separating migrant children from their parents. "It's not easy, but populism is not the solution," the Pope said at the time.

During his visit to Rabat, Morocco's capital, Pope Francis offered his support to the country's tiny Christian population.

He began the day visiting Spanish nuns who provide medical care to children and teach women to read; he then spoke at the Cathedral of Rabat, where he greeted a 95-year-old monk who survived a 1996 massacre in Algeria.