Fionnuala Ni Aolain said that during a visit to the Kurdish-controlled camps, she had noted "the separation of hundreds of adolescent boys from their mothers without any legal procedure".

This appeared in particular to be happening to so-called third-country nationals, from countries other than Syria and Iraq.

This is "summary separation based on an unproven security risk that male children pose upon reaching the age of adolescence," she told journalists in Geneva after a five-day visit to Syria.

"Every single boy child I met was clearly traumatised by the separation from their mothers," she said, adding that she had seen separated boys as young as 11.

"This systematic practice of enforced separation ... is in clear violation of international law".

Ni Aolain, the special rapporteur on the promotion and protection of human rights while countering terrorism, is the first UN rights expert to have gained access to detention camps and prisons in the northeast.

- Repatriation 'urgently' needed -

An estimated 52,000 people -- nationals from 57 countries -- are still reportedly being held in the squalid and overcrowded al-Hol and al-Roj detention camps for jihadists and their family members in Kurdish-administered territory.

Sixty percent of those there are children, mostly under the age of 12, said Ni Aolain.

She acknowledged the intense political and security complexity of the situation on the ground.

But that in no way justified "the mass indefinite and arbitrary detention of children, particularly boys, in various types of facilities", she added.

Their repatriation needed to happen "urgently", she insisted, stressing that besides the US-backed local authorities, all the home countries bore responsibility for the rights violations taking place.

Western governments have faced mounting criticism for not taking back more of their citizens who travelled to Iraq and Syria to volunteer for the Islamic State group.

Ni Aolain acknowledged that there had been "some positive movement on repatriation" -- with numbers having come down from a high of over 70,000.

But she warned that at the current rate, "these facilities will on average, at a minimum, stay in business for 20 years".