The Trump administration policy has been heavily criticised

Audio has emerged of children crying at a migrant processing centre on the US-Mexico border.

The recording features boys and girls separated from their parents as part of the Trump administration's 'zero tolerance' crackdown on those entering the US illegally.

Shouts of "Papa!... Mami!" can also be heard, while one child repeatedly pleads for someone to call her aunt, whose telephone number she has memorised and recites to officials.

Human rights attorney Jennifer Harbury said she received the tape from a whistleblower and told ProPublica it was recorded last week, but did not say which camp.

Agents running the facilities have said everyone detained is given adequate food, access to showers and laundered clothes, as well as medical care.

But politicians and religious groups are also among those to have criticised the Trump administration's "inhumane" policy.

The policy has been widely condemned, with the latest criticism coming from the French government which says it doesn't share the same values.

Former director of the CIA, Michael Hayden, tweeted a black and white photo of Birkenau, the Nazi concentration and death camp in Poland, and wrote: "Other governments have separated mothers and children."

Politics professor Scott Lucas says it's a hugely divisive issue.

"I'm not exaggerating here - this is the most serious time for the United States since 1945 - that this threat that is posed to American values, the threat that is posed to the American system and the threat that is posed to Americans is great.

"Bur at the end of the day, there is a system in which many Americans can speak out and are speaking out against this".