Heartbreaking audio of wailing children desperately calling out for their parents at the Mexico-US border has been released, highlighting the human toll of Donald Trump’s harsh immigration stand-off.

The nearly eight-minute clip recorded 10 children from Central America (one as young as six) who have been separated from their parents, some screaming “mami” and “papá” over and over.

The audio of the children, who were detained last week at the border by immigration authorities, has sparked sadness and outrage at US President Donald Trump’s “zero tolerance” policy that arrests all adults caught trying to enter the US illegally – including those seeking asylum.

At one point in the clip, a six-year-old El Salvadorian girl begs a consulate worker to call her aunt, whose number she had memorised in case she was separated from her family.

“My mommy says that I’ll go with my aunt and that she’ll come to pick me up there as quickly as possible,” the unidentified girl says, according to the translation.

But a US Border Patrol agent can also be heard in the recording taunting the children by making fun of their crying.

“Well, we have an orchestra here,” the agent says. “What’s missing is a conductor.”

WARNING: Some readers may find parts of the audio disturbing

The recording, published by non-profit media outlet ProPublica, has not yet been authenticated. The person who made it reportedly asked not to be identified out of fear of retaliation.

President Trump said on Tuesday morning (AEST) he would not allow the US to become a “migrant camp”, as his administration faced a storm of criticism for separating the children from their parents at the border.

Democrats, and some in Mr Trump’s own Republican Party, have blasted the administration for separating nearly 2000 children from their parents at the border between mid-April and the end of May.

Former first lady Laura Bush called the separation policy cruel and immoral, while Republican Senator Susan Collins expressed her concern.

Even First Lady Melania Trump appeared to question her husband’s policy on Sunday, while medical professionals have argued the practice could cause lasting trauma to children.

Video footage released by the government showed migrant children held in wire cages and sitting on concrete floors.

BREAKING: Border Patrol @CBP just gave us this video of the detention facility we toured yesterday in McAllen, Texas. We weren't allowed to bring in cameras, or interview anyone. To be clear: this is government handout video. pic.twitter.com/Zjy80qIZFZ

— David Begnaud (@DavidBegnaud) June 18, 2018

Mr Trump, who has made a tough stance on immigration a major goal of his presidency, responded sharply to critics on Monday.

The United States will not be a migrant camp, and it will not be a refugee holding facility. It won’t be.

“You look at what’s happening in Europe, you look at what’s happening in other places – we can’t allow that to happen to the United States, not on my watch,” Mr Trump said. He was speaking at the White House while announcing an unrelated policy.

More than 2000 children have been separated from their families since ‘zero tolerance’ on migrants was declared. Photo: US Customs and Border Patrol

Having consistently blamed Democrats for the impasse, Mr Trump tweeted early Tuesday (AEST): “It is the Democrats (sic) fault for being weak and ineffective with Boarder (sic) Security and Crime.”

United Nations Secretary-General Antonio Guterres has also weighed in, saying refugee and migrant “children must not be traumatised by being separated from their parents”.

In Geneva, the UN’s High Commissioner for Human Rights, Zeid Ra’ad al-Hussein, called for the Trump administration to halt the practice, saying it was unconscionable that any country would seek to deter parents from migrating “by inflicting such abuse on children”.

-with AAP