President Donald Trump’s threat to close the southern border was met with criticism and concern from politicians, activists, the public—and Pope Francis. 

Trump is taking the action, he said, because of what he believes is a crisis in drug smuggling and human trafficking.

In remarks to reporters Sunday on the papal plane en route from Morocco to Rome, Pope Francis said that Trump—and people who think the way the U.S. president does about migrants—are ultimately trapping themselves in an inhumane situation. 

“Builders of walls, be they made of razor wire or bricks, will end up becoming prisoners of the walls they build,” the pope said. 

The comments preceded a tweet made by the pope Saturday in which he celebrated the rights of migrants to self-determination.

“Every human being has the right to life, to dream and to find his or her rightful place in our ‘common home!'” posted the pontiff. “Every person has a right to the future.”

The measure to close the border comes as the administration plans to cut aid to Central American countries—a move that experts argue is almost certain to increase, not curtail, the number of immigrants at the border. 

Acting Chief of Staff Mick Mulvaney, speaking to ABC News anchor Jon Karl Sunday, said that unless “something dramatic” changed, the closure would happen this week.

Mulvaney pushed back on the idea that an increase in aid would lead to lower numbers of people on the border in an interview with CNN‘s Jake Tapper, also aired Sunday. 

“If we’re going to give these countries hundreds of millions of dollars, we would like them to do more,” said Mulvaney. “If it’s working so well why are the people still coming?”

Whether or not Trump actually goes through with his threat is, as with most actions the president proposes, an open question. Rep. Tom Kole (R-Okla.) told MSNBC‘s Hallie Jackson Monday that Trump may be bluffing. 

Democrats took a hard line against the proposal.