Pope Francis has criticised the Trump administration's policy of separating migrant families at the Mexican border, saying populism is not the answer to the world's immigration problems.

Speaking to Reuters, the Pope said he supported recent statements by US Catholic bishops who called the separation of children from their parents "contrary to our Catholic values" and "immoral".

"It's not easy, but populism is not the solution," Francis said.

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The policy has caused an outcry in the US and has been condemned abroad as videos emerged of youngsters held in concrete-floored enclosures and an audio of wailing children went viral.

US Catholic bishops have joined other religious leaders in the United States in condemning the policy.

"I am on the side of the bishops' conference," the pope said, referring to two statements from US bishops this month.

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"Let it be clear that in these things, I respect [the position of] the bishops conference."

The pope said populists were "creating psychosis" on the issue of immigration, even as aging societies like Europe faced "a great demographic winter" and needed more immigrants.

Without immigration, he added, Europe "will become empty".

This is wrong: Theresa May

British Prime Minster Theresa May also entered the fray on Wednesday, saying images from the US of migrant children being held in cages were "deeply disturbing".

"On what we have seen in the United States, pictures of children being held in what appear to be cages are deeply disturbing... This is wrong," she told MPs.

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May said she would raise the issue with Trump when the pair meet in Britain next month.

"When we disagree with the United States we tell them so," she told MPs.

Additional reporting: AFP