HARROWING audio recordings that appear to capture the voices of children crying for their parents have provoked outrage over US President Donald Trump’s “zero tolerance” immigration policy.

One child is heard crying “Papa, papa” in the sound file which is claimed to have been recorded at a US immigration detention centre on the Mexican border.

Pressure is mounting on Mr Trump over his policy that sees migrant children held in detention centres apart from their parents after human rights lawyer Jennifer Harbury said she was leaked the recording from a whistleblower last week.

Politicians visited a Texan warehouse on Sunday where hundreds of children are being held in cages after being separated from their families when they try to enter the US. One of the cages held 20 children.

Homeland Security Secretary Kirstjen Nielsen insisted that children taken into custody by the government are being treated humanely.

But United Nations High Commissioner for Refugees Filippo Grandi said the agency had expressed concern over the policy to Washington, warning it is “unacceptable” in any circumstance.

Former CIA boss Michael Hayden defended his comparison of Mr Trump’s policy to the treatment of concentration camp prisoners in Nazi Germany, saying he “wanted to grab people’s attention.”

Mr Hayden tweeted a picture of Auschwitz on Saturday with the caption “other governments have separated mothers and children.”

Despite the growing international outrage Mr Trump has continued to blame the Democrats for the separation of migrant children from their parents and yet remains defiant over his “zero tolerance” immigration policy.

“The United States will not be a migrant camp and it will not be a refugee holding facility. Not on my watch,” he remarked defiantly on Monday.