The UN human rights chief on Monday urged Washington to stop separating migrant children from their parents at the US border, describing the policy as "unconscionable".
"The thought that any state would seek to deter parents by inflicting such abuse on children is unconscionable," Zeid Ra’ad Al Hussein said as he opened a session of the UN Human Rights Council in Geneva.
The "zero-tolerance" border security policy implemented by President Donald Trump’s administration has sparked global outrage.
Images released on Monday morning showed segregated children detained in cages inside a warehouse in Texas, after journalists were given access for the first time.
It came as senior Republicans continued to call into question the hardline policy. Melania Trump, the first lady, also appeared to question the treatment of migrants. In a statement last night she said she "hates to see children separated from their families", while the former first lady Laura Bush said the policy "breaks my heart".
The government has said that during one recent six-week period nearly 2,000 minors were separated from their parents or adult guardians.
The number of separations has jumped since early May, when Attorney General Jeff Sessions announced that all migrants illegally crossing the US border with Mexico would be arrested, regardless of whether the adults were seeking asylum.
Since children cannot be sent to the facilities where their parents are held, they are separated.
Zeid quoted the American Association of Pediatrics as describing the practice as "government-sanctioned child abuse" which may cause "irreparable harm," with "lifelong consequences".
"I call on the United States to immediately end the practice of forcible separation of these children," he said, urging Washington to ratify the Convention of the Rights of the Child.
The US is the only country that has not ratified the convention.
Ratification, Zeid said, would "ensure that the fundamental rights of all children, whatever their administrative status, will be at the centre of all domestic laws and policies."
Zeid’s address at the start of the 38th session of the UN Human Rights Council marks his last address to the body before he is due to step down at the end of August.
The session kicked off under a cloud of growing US criticism of the council. Diplomatic sources said there was a risk that Washington may withdraw from the council altogether.
The US Border Patrol on Sunday allowed reporters to briefly visit the facility where it holds families arrested at the border.
Inside an old warehouse in South Texas, hundreds of children wait in a series of cages created by metal fencing. One cage had 20 children inside. Scattered about are bottles of water, bags of chips and large foil sheets intended to serve as blankets.
One teenager told an advocate who visited that she was helping care for a young child she didn’t know because the child’s aunt was somewhere else in the facility. She said she had to show others in her cell how to change the girl’s diaper.
Mr Trump himself has also said he "hates to see separation of parents and children" and has accused Democrats of pursuing a "horrible and cruel legislative agenda".
The Border Patrol said close to 200 people inside the facility were minors unaccompanied by a parent. Another 500 were "family units," parents and children. Many adults who crossed the border without legal permission could be charged with illegal entry and placed in jail, away from their children.
Reporters were not allowed by agents to interview any of the detainees or take photos.
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Stories have spread of children being torn from their parents’ arms, and parents not being able to find where their kids have gone. A group of congressional lawmakers visited the same facility on Sunday and were set to visit a longer-term shelter holding around 1,500 children – many of whom were separated from their parents.
"Those kids inside who have been separated from their parents are already being traumatised," said Democratic Sen. Jeff Merkley of Oregon, who was denied entry earlier this month to children’s shelter. "It doesn’t matter whether the floor is swept and the bedsheets tucked in tight."
In Texas’ Rio Grande Valley, the busiest corridor for people trying to enter the US, Border Patrol officials argue that they have to crack down on migrants and separate adults from children as a deterrent to others.
"When you exempt a group of people from the law … that creates a draw," said Manuel Padilla, the Border Patrol’s chief agent here. "That creates the trends right here."
Agents running the holding facility – generally known as "Ursula" for the name of the street it’s on – said everyone detained is given adequate food, access to showers and laundered clothes, and medical care. People are supposed to move through the facility quickly. Under US law, children are required to be turned over within three days to shelters funded by the Department of Health and Human Services.
Padilla said agents in the Rio Grande Valley have allowed families with children under the age of 5 to stay together in most cases.
An advocate who spent several hours in the facility on Friday said she was deeply troubled by what she found.
Michelle Brane, director of migrant rights at the Women’s Refugee Commission, met with a 16-year-old girl who had been taking care of a young girl for three days. The teen and others in their cage thought the girl was two years old.
"She had to teach other kids in the cell to change her diaper," Brane said.