(CNN) Pope Francis has added his voice to those criticizing the Trump administration's "zero tolerance" policy that has resulted in the separation of families at the Mexican border.

In a wide-ranging interview with Reuters at his Vatican residence published on Wednesday, World Refugee Day , Francis said he supported the statements made by US Catholic bishops who called the separation of children from their parents "immoral" and "contrary to our Catholic values."

"I am on the side of the Bishops' Conference," Francis told Reuters, referring to those statements. "It's not easy, but populism is not the solution," he added.

As harrowing images and audio of children emerged from the border this week, Trump has come under increasing pressure to drop his "zero tolerance" policy. But Trump has defended his administration's actions and falsely blamed the Democrats for the continuing separations.

The Pope added that it was not solely "a Trump issue."

"During the Obama administration, I celebrated mass at Ciudad Juarez on the border," he said, referring to his visit to the Mexican border city in 2016. "The problem already existed then. It's not just a Trump issue but also one of the previous governments."

In an interview with Reuters, Pope Francis criticized the Trump administration on a number of issues.

'You cannot reject people who arrive'

In the rare interview, Francis, who has previously come under fire from more conservative cardinals for his teachings, defended his more liberal, outspoken approach to moral and social issues.

He insisted that the future of the Catholic Church was "on the street" and said he wanted to appoint more women to lead Vatican departments because they were better at resolving conflicts.

He also directly criticized Trump's decisions to pull out of a climate change accord and to implement new restrictions on American travel and trade with Cuba last year.

But it was the subject of immigration that dominated the discussion, with the pontiff also taking aim at the response of some European governments to migrants and refugees arriving on the continent.

"I believe that you cannot reject people who arrive. You have to receive them, help them, look after them, accompany them and then see where to put them, but throughout all of Europe," Francis said.

"Some governments are working on it, and people have to be settled in the best possible way, but creating psychosis is not the cure," he added. "Populism does not resolve things. What resolves things is acceptance, study, prudence."

Theresa May: 'Deeply disturbing'

British Prime Minister Theresa May joined the condemnation of the Trump administration's "zero tolerance" policy on Wednesday, describing the detention of children in cages as "deeply disturbing."

Asked in parliament whether she would discuss immigration with Trump when he visits the UK in July, May said they would speak about "a range of issues" and that "when we disagree with them, we say so."

"The pictures of children being held in what appear to be cages are deeply disturbing," she said. "This is wrong, this is not something that we agree with, and this is not the United Kingdom's approach."

On Tuesday, Irish Taoiseach Leo Varadkar said he planned to convey his government's "grave concern" about the controversial practice to the acting US ambassador.