UNITED NATIONS (AFP) - In a rare criticism of the United States, UN Secretary-General Antonio Guterres said Monday (June 18) that migrant children should not be separated from their families at the US-Mexico border.

"As a matter of principle, the secretary-general believes that refugees and migrants should always be treated with respect and dignity, and in accordance with existing international law," said UN spokesman Stephane Dujarric.

"Children must not be traumatised by being separated from their parents. Family unity must be preserved," he added.

UN rights chief Zeid Ra'ad Al Hussein also spoke out against the US administration's policy of separating migrant families, saying "the thought that any state would seek to deter parents by inflicting such abuse on children is unconscionable."

About 2,000 children have been separated from their parents or adult guardians over six weeks, according to US authorities.

The number of separations has jumped since early May, when Attorney General Jeff Sessions announced that all migrants illegally crossing the US border with Mexico would be arrested, regardless of whether the adults were seeking asylum.

Since children cannot be sent to the facilities where their parents are held, they are separated, which the American Academy of Pediatrics has warned causes "irreparable harm" to the children.