UN High Commissioner for Human Rights Michelle Bachelet voiced concern over the move by the administration of President Donald Trump last month to allow migrant children and their families to be detained for unlimited periods.
She pointed to the Convention on the Rights of the Child, which states that children can be detained only "as a measure of last resort and for the shortest appropriate period of time."
"If they are going to make it indefinite, that is much worse," she told reporters in Geneva.
"It's against all the legal conventions and international human rights law and the laws for the child."
Her comments came after the Department of Homeland Security on August 22 announced that it was terminating the 1997 Flores Settlement Agreement, a legal ruling that said the government could hold migrant children in detention for no more than 20 days.
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The White House said the Flores rule was outdated and did not take into account the massive increase in Central American migrant families and children crossing into the United States in recent years.
A new policy, which was scheduled to be implemented within weeks but which has already met numerous legal challenges, will allow indefinite detention of migrant children.
Trump made cracking down on illegal immigration a key part of his 2016 presidential campaign platform.
In 2018, he launched a "zero tolerance" policy under which more than 2,300 children were separated from their parents at the border, before the government backed down amid a massive public outcry.
But Bachelet voiced concern Wednesday at information that hundreds of children had been separated from their families since the "zero tolerance" policy ended.
"We do believe that the arbitrary separation of families constitutes an arbitrary and unlawful interference with family life and a grave violation of the rights of child," she said.
The UN rights chief also took issue with the Trump administration's new "public charge" rule aimed at denying permanent residency and citizenship to immigrants who claim welfare benefits.
"This new measure raises for us several concerns... including that legally resident migrants would withdraw from necessary public assistance programmes, including medical, housing and food programmes because they would fear to lose their documents," she said.
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