UN human rights office says implementation of policy on US-Mexico amounts to a violation of child rights.

The UN human rights office is calling on the administration of US President Donald Trump to “immediately halt” its policy of separating children from their parents after crossing the US border with Mexico.

Spokeswoman Ravina Shamdasani scolded the US for its “zero tolerance” policy in which some families have been separated after crossing the border.

The rights office said in a statement that “children should never be detained for reasons related to their own or their parents’ migration status. Detention is never in the best interests of the child and always constitutes a child rights violation.”

Shamdasani told a UN briefing Tuesday that the practice of separating families amounted to an “arbitrary and unlawful” interference in family life, calling it a “serious violation” of the rights of children, the Associated Press reported.

Hundreds of children have been separated from their parents, who were detained after making the crossing.

Thousands more have been detained after entering the US alone.

Officials say that around 1,500 children who arrive at the border by themselves are unaccounted for, according to figures first reported by the New York Times and the Associated Press last month.

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The scale of those numbers has sparked concern that some of the children may have fallen into the hands of human traffickers.

The Trump administration has beefed up security on the US-Mexico border and has vowed to prosecute refugees and migrants who attempt to cross it.

“If you cross the border unlawfully .. then we will prosecute you,” Attorney General Jeff Sessions said earlier this month.

“If you’re smuggling a child, then we’re going to prosecute you, and that child will be separated from you, probably, as required by law,” he added.

“If you don’t want your child separated, then don’t bring them across the border illegally. It’s not our fault that somebody does that.”

The policy marks a break with the previous precedent, which allowed families to stay together pending deportation.

Politicians from the Democratic Party have called the policy one of Trump’s “most inhumane” and “cruel”.