The United Nations has issued a damning condemnation of Donald Trump's policy that saw migrant children separated from their parents at the border, suggesting it "may amount to torture".

In a statement issued by the UN's Human Rights Council, experts said the president's recent executive order, ostensibly to halt the controversial separations, failed to resolve the problem and "may lead to indefinite detention of entire families in violation of international human rights standards".

“This executive order does not address the situation of those children who have already been pulled away from their parents. We call on the government of the US to release these children from immigration detention and to reunite them with their families based on the best interests of the child, and the rights of the child to liberty and family unity,” the group of 11 experts said.

“Detention of children is punitive, severely hampers their development, and in some cases may amount to torture,” the experts said. “Children are being used as a deterrent to irregular migration, which is unacceptable.”

A number of rights groups have questioned Mr Trump's order, issued on Wednesday, mostly for offering few details on how to deal with the more than 2,300 children detained by the US government since the "zero tolerance" policy was enacted by the president's administration in mid-April.

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That zero tolerance policy means all adults who cross the border illegally are to be prosecuted, even those who make credible fear claims to seek asylum. The government has taken any children of individuals facing prosecution into their care while cases are processed. Mr Trump's executive order sought to end that separation, but did not indicate that adults seeking asylum would not continue to be prosecuted.

Mr Trump said on Thursday that he had ordered authorities to begin the process of re-uniting children with their parents, and the Justice Department asked a federal court for dispensation to hold children longer than the 20 days currently allowed - in order to help reunite families.

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However, there are still few concrete details about how any reunifications will happen.

"The separations have been conducted without notice, information, or the opportunity to challenge them," the experts said in the statement. "The parents and children have been unable to communicate with each other. The parents have had no information about the whereabouts of their children, which is a cause of great distress. Moreover, we are deeply concerned at the long-term impact and trauma, including irreparable harm that these forcible separations have on the children".

The UN experts noted that there are exacerbating circumstances for some of the children that makes their removal from family especially damaging. Those concerns include worries about children with disabilities who may need specialised support, and that some of the children may still be breastfeeding.

The zero tolerance policy was announced earlier this year by Attorney General Jeff Sessions, and the first six weeks saw those thousands of separations.

Accompanying the disclosure of the number of children who had been separated from their parents came shocking images of the facilities in which those youngsters were being held. In at least one Texas facility, children were pictured in metal cages inside of a temperature controlled facility. Politicians and the media pressed for access to the facilities to monitor the conditions there, and in some cases were denied entrance. The children in those facilities were provided with small sleeping pads, and foil blankets for warmth.

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The UN condemnation comes just days after the United States withdrew its membership from the Human Rights Council, saying that the council is against Israel.

"For too long, the Human Rights Council has been a protector of human rights abusers, and a cesspool of political bias," Nikki Haley, the US ambassador to the UN, said Tuesday during a speech at the State Department in Washington. She continued to say that the US had withdrawn from the council to reaffirm America's commitment to human rights, which she said "does not allow us to remain a part of a hypocritical and self-serving organisation that makes a mockery of human rights".