Francis was asked by reporters about the proposed border wall on the way to Panama, where he is looking to leave the sex abuse scandals buffeting his papacy behind.

"It is the fear that makes us crazy," he responded.

Francis, the Roman Catholic Church's first Latin American pope and the son of Italian immigrants to Argentina, has made the plight of migrants and refugees a cornerstone of his papacy.

He is also expected to offer words of encouragement to young people gathered in Panama for World Youth Day, the church's once-every-three-year rally that aims to invigorate the next generation of Catholics in their faith.

Panama Archbishop Jose Domingo Ulloa said Francis's message is likely to resonate with young Central Americans who see their only future free of violence and poverty in migrating to the US - "young people who often fall into the hands of drug traffickers and so many other realities that our young people face".

The Pope is expected to urge young people to create their own opportunities, while calling on governments to do their share as well.

The visit is taking place as the US government remains partly shut down in a stand-off between the Trump administration and Democrats over funding for Trump's promised border wall.

Francis has famously called for "bridges, not walls".

After celebrating mass in 2016 on the Mexican side of the US border, he denounced anyone who wants to build a wall to keep out migrants as "not Christian".

Australian Associated Press