“There are almost 57,000 unaccompanied children in the United States. I’m particularly concerned because the United States appears to be taking steps to deport most of these children,” Pillay told reporters.
The arrival of huge numbers of unaccompanied children has overwhelmed US authorities lacking the financial and legal means to curb the illegal influx.
President Barack Obama has insisted new arrivals will be sent home, and US lawmakers introduced a plan earlier this month to speed up repatriation.
Washington has declared the influx a humanitarian crisis. Most of the children come from El Salvador, Guatemala and Honduras where youths are fleeing poverty and gang violence.
Pillay said victims should be offered a haven and cared for.
“Deportation should only take place if their protection is guaranteed at the places they are being sent to,” she said.
“The real causes of the crisis, in the countries of origin and destination, need to be tackled. Criminal trafficking networks have to be dismantled and punished. It’s the traffickers, after all, not the victims or their families, that must be punished,” she added.
If minors are alone and do not hail from neighbouring Mexico or Canada, US authorities are required to provide them shelter and move them within 72 hours to housing where they can then receive medical care and legal aid services.
They spend 34 days on average in such housing before being released, in 85 percent of the cases to a relative already present in the United States.
They are then ordered to appear in immigration courts where their cases are heard.
But earlier this month about 2,000 children were still in detention centres operated by border patrol agents beyond the 72 hour limit.
“I recognise that there’s a complicated political situation,” said Pillay.
“But I state categorically that detention of migrants for immigration purposes should be a last resort option. It should be proportionate and only permissible for the shortest period of time,” she said.
“Detention of children constitutes a violation of their rights because it contravenes the principle of the best interests of the child.”
US officials told senators earlier this month that authorities are spending $250 to $1,000 per child per day for housing and care.