The top U.N. human rights official called on the United States on Monday to halt its "unconscionable" policy of forcibly separating children from migrant parents.

"The thought that any state would seek to deter parents by inflicting such abuse on children is unconscionable," Zeid Ra'ad Al Hussein said as he opened a session of the UN Human Rights Council in Geneva.

Zeid quoted the American Association of Pediatrics as describing the practice as "government-sanctioned child abuse" which may cause "irreparable harm," with "lifelong consequences". "I call on the United States to immediately end the practice of forcible separation of these children," he said, urging Washington to ratify the Convention of the Rights of the Child. The US is the only country that has not ratified the convention.

"ensure that the fundamental rights of all children, whatever their administrative status, will be at the centre of all domestic laws and policies." Zeid's address at the start of the 38th session of the UN Human Rights Council marks his last address to the body before he is due to step down at the end of August.

US Federal department announced last May 6 that it will file criminal charges against all foreigners who are caught crossing the border illicitly.

That measure, known as "zero tolerance," sends parents to jail and the children to refuges financed with federal funds and supervised by the Department of Health and Human Services.

At least 600 children were separated from their parents last month.

Thousands of people poured into the streets Friday in some 30 cities around the US to protest against the growing number of children separated from their parents at the Mexican border as a result of the zero tolerance policy promoted in May by US Attorney General Jeff Sessions.

Though the demonstration in Washington was the main one, throughout the day there were numerous protests in some 30 US cities including Los Angeles, New York and Chicago, and internationally in Barcelona, Spain, according to the organizers.