An audio recording in which migrant children in the US can be heard crying for their parents has been released as US President Donald Trump remains defiant over his immigration policy.
Some 2,000 migrant children have been separated from their parents in the past few months after their families tried to cross the US border illegally.
Mr Trump said he would not allow the US to become a "migrant camp".
The UN high commissioner for refugees says the policy is "unacceptable".
The recording was released by investigative US media outlet ProPublica on Monday, and was said to have been made from inside a US customs and border protection facility.
In it, several Central American children separated from their parents and thought to be aged between four and 10 can be heard sobbing and wailing, and calling for their mother or father.
One border patrol agent can be heard saying: "We have an orchestra here. What's missing is a conductor."
The UN High Commissioner for Refugees, Filippo Grandi, has told the BBC the Trump administration's "zero-tolerance" policy towards illegal immigration is unacceptable.
"It is absolutely wrong to separate children from their families in any circumstances and particularly when people are in distress as, for example, people that are fleeing from violence and persecution, as is the case for people in Central America," he said.
"So, we are advising the government of the United States that this is not the right way to handle that particular phenomenon of people seeking asylum across the border."
On Monday, Mr Trump said the US would "not be a migrant camp", or a refugee-holding facility.
"You look at what's happening in Europe, you look at what's happening in other places.
"We cannot allow that to happen to the United States. Not on my watch."
The Republican president blamed Democrats for not coming to the table to negotiate immigration legislation.
Meanwhile, Mr Trump's job approval rating averaged 45%, according to Gallup polling conducted last week. This matches his previous peak, in the first week after his inauguration in January 2016.
What's the policy?
Nearly 2,000 children have been separated from their parents at the border between mid-April and the end of May.
The administration's "zero-tolerance" policy entails criminally charging those entering the US illegally, including asylum seekers.
This has led to parents being separated from their children, who are not charged with a crime.
As a result, hundreds of children are being housed in detention centres, including warehouses and converted supermarkets.
Images from some of the border camps show children being held in caged areas separated by chain-link fencing.
Some shelters and foster homes have said they are running out of space.
Officials have also announced plans to erect tent cities that will hold hundreds more children in the Texas desert where temperatures regularly reach 40C (105F).
Who's criticising it?
Democrats and some in Mr Trump's own Republican Party have strongly condemned the administration.
The crackdown has even provoked fierce criticism from the president's wife, Melania Trump, who said over the weekend she "hates to see children separated from families".
Former First Lady Laura Bush meanwhile wrote in a Washington Post op-ed that the tactics were "immoral" and evoked Japanese-American internment camps during the Second World War.
Sometimes truth transcends party. https://t.co/TeFM7NmNzU— Michelle Obama (@MichelleObama) June 18, 2018
Former Democratic presidential nominee Hillary Clinton said Mr Trump's widely debunked claims that the family separations were mandated by law was "an outright lie".
Most congressional Republicans, however, have declined to criticise Mr Trump over the policy.
Who's defending it?
On Monday, US Homeland Security Secretary Kirstjen Nielsen defended the "zero-tolerance" policy, arguing that the government was simply following the law and it would be up to Congress to change it.
During a heated briefing at the White House, Ms Nielsen also insisted that migrants who attempt to cross the US border were putting their children in an unsafe situation.
Fox News host Laura Ingraham called the detention centres "essentially summer camps", and praised Mr Trump for "doing what we should have been doing all along, prosecuting all border crashers".
Conservative author Ann Coulter called the children "child actors weeping and crying" on TV networks.
"Do not fall for it, Mr President," she warned in an interview on Fox News.
More on US immigration
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