Family members of suspected ISIL fighters, including thousands of children, need to be returned to their home countries, said the United Nations human rights chief Michelle Bachelet.

Among those stateless relatives of suspected terrorists, children in particular, have endured "grievous violations" of their rights, she said.

"Foreign family members should be repatriated, unless they are to be prosecuted for crimes in accordance with international standards," she said at the UN Human Rights Council's 41st session in Geneva.

Bachelet stated that more than 55,000 men, women and children have been stateless since the self-styled "Caliphate" fell in spring. 

"While the majority of these individuals are Syrian or Iraqi, they also include alleged foreign fighters from nearly 50 countries," she added, noting that at least 11,000 suspected family members of foreign ISIL fighters are still being held at Al Hol camp in northeastern Syria "in deeply sub-standard conditions."

According to the UN Children's Fund (UNICEF), it's estimated that there are 29,000 children of foreign fighters in Syria, of whom two-thirds originally are from Iraq.

During years of conflict, thousands of children have been born to foreign families, Bachelet noted before appealing to member states to give them nationality, or else risk fueling a "narrative of grievance and revenge."

She added that "well over" 150 men and women have been sentenced to death in Iraq under anti-terror laws, which conduct trials without the due process guarantees provided, and called on states of origin to "make all efforts to ensure that they will be treated in accordance with international law."