US President Donald Trump last week abrogated an order issued by his predecessor Barack Obama that protected around 800 000 young people by granting them temporary legal status as part of the Deferred Action for Childhood Arrival programme (DACA).
The move gives the so-called "Dreamers", many of them Hispanic who are now in their twenties, somewhere between six and 24 months before they become illegal and subject to potential deportation.
"I am concerned by the government's decision to end the Deferred Action for Childhood Arrivals programme," UN rights chef Zeid Ra'ad Al Hussein said at the opening of the 36th session of the Human Rights Council in Geneva.
He pointed to evidence of the programme's "positive impact on the lives of hundreds of thousands of young migrants, and on the US economy and society," and said he hoped Congress would now act to provide former DACA beneficiaries with "durable legal status".
Trump's move fulfilled a campaign promise of zero tolerance toward illegal immigrants.
But the president insisted he was showing compassion for the Dreamers, many of whom know only the United States and speak only English, by giving Congress six months to craft a lasting legal solution.
Zeid also voiced alarm at an "increase in detentions and deportations of well-established and law-abiding immigrants" in the United States.
He said the number of detentions of migrants with no criminal convictions was 155% higher during the first five months of this year than during the same period of 2016.
"Some migrants, including longstanding residents, are now so frightened of expedited deportation they refrain from accessing police protection and courtrooms," Zeid said.
He said reports of rape filed by Latina women in the city of Houston fell by 43% in the first three months of 2017.
Zeid also reiterated his concerns over the anti-semitism and racism on display last month in Charlottesville, Virginia, and also "increasingly manifested online and in public debates".
"Free speech is an invaluable and essential right, under both international standards and US law, and it should not be weaponised by calls for violence and hatred," he said.
Last month, the UN rights chief said Trump was largely to blame for the deteriorating discourse in the country, warning among other things that his relentless attacks on the media could trigger violence against journalists.