Speaking at the opening of a regular Human Rights Council session in Geneva, Zeid Ra’ad al-Hussein urged the US authorities to end a “zero tolerance” policy that has seen almost 2,000 children taken from their families in the past six weeks.
“The thought that any state would seek to deter parents by inflicting such abuse on children is unconscionable,” said Mr Hussein.
Mr Hussein, a Jordanian prince, is due to stand down from his position in August.
He used his remarks on Monday to drawn attention to a number of countries, including Syria, Myanmar, Hungary, Nicaragua, Israel, North Korea, and India- and Pakistan-controlled Kashmir.
Yet, according to the Associated Press, he also took aim at nationalist agendas around the world being promoted by “self-serving, callous leaders”.
In remarks that some will assume were aimed at Donald Trump, Mr Hussein said “the more pronounced their sense of self-importance, the more they glory in nationalism, the more unvarnished is the assault on the overall common good - on universal rights, on universal law and universal institutions, such as this one”.
The criticism from Mr Hussein came as Mr Trump found himself under increasing pressure to explain a policy taken up in April that meant everyone caught trying to illegally cross US-Mexico the border was immediately charged with federal offences, something that obliged parents to be separated from their families. People applying for asylum were treated no differently.
As images have emerged of youngsters in tent camps and holding facilities made up of cages, the groundswell of opposition to the policy has increased. Over the weekend, Mr Trump’s own wife, First Lady Melania Trump, said she “hates to see children separated from their families”.
Meanwhile, former first lady Laura Bush, whose husband sought to agree a deal on immigration reform when he was president, wrote an article for the Washington Post in which she voiced rare public criticism of a man her family is known to be at adds with.
“I live in a border state. I appreciate the need to enforce and protect our international boundaries, but this zero-tolerance policy is cruel. It is immoral. And it breaks my heart,” she wrote.
Mr Trump, who is due to visit Capitol Hill on Tuesday to discuss two possible immigration bills, has claimed, inaccurately, the policy was established by the previous administration and he can do nothing about it without the cooperation of Democrats to completely overhaul immigration. Critics say Mr Trump is seeking to use the children as leverage as he seeks to push legislators to provide funds to pay for a border wall.
He continued this argument on Monday, tweeting: "Why don’t the Democrats give us the votes to fix the world’s worst immigration laws? Where is the outcry for the killings and crime being caused by gangs and thugs, including MS-13, coming into our country illegally?"
While some children were separated by their families during the administrations of Barack Obama and George Bush, last month Attorney General Jeff Sessions announced that officials would start prosecuting all people who tried to cross the border illegally. The new approach meant the Department of Justice would prosecute everyone crossing, even those seeking to make asylum claims.
As a result of the shift, people travelling with children were detained rather than being charged and released. The children were not charged, but were held separately.
Mr Sessions last week used a passage from the bible to defend his department’s policy.
“I would cite you to the Apostle Paul and his clear and wise command in Romans 13, to obey the laws of the government because God has ordained the government for his purposes,” he said during a speech to police officers in Indiana.
“Orderly and lawful processes are good in themselves. Consistent and fair application of the law is in itself a good and moral thing, and that protects the weak and protects the lawful."
On Monday, Mr Trump’s one-time communications director, Anthony Scaramucci says it “doesn’t feel right” for the Trump administration to blame Democrats for separating children. Using the separation of children and parents “as a leverage point or a negotiating point...just doesn't feel right,” he told CNN.
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