The sprawling and overcrowded Kurdish-run Al-Hol camp in northern Syria is home to more than 50,000 people, including family members of suspected Islamic State group militants as well as displaced Syrians and Iraqi refugees.

Nearly half of the camp's population is under the age of 12 and residents are "deprived of their rights, vulnerable, and marginalised", Guterres said in a statement during a visit to Iraq.

"They are trapped in a desperate situation with no end in sight," he said at the Jadaa camp, which Iraqi authorities describe as a "rehabilitation" centre for those returning from Syria.

"I have no doubt to say that the worst camp that exists in today's world is Al-Hol, with the worst possible conditions for people and with enormous suffering for the people that have been stranded there for years," Guterres said.

He urged UN member states who have nationals in Al-Hol to "significantly step up their efforts to facilitate the safe and dignified repatriation of their nationals".

He praised war-ravaged Iraq, which has repatriated hundreds of families from Al-Hol since May 2021.

"All countries with their citizens in Al-Hol must do the same, and must do the same in a dignified repatriation in line with applicable international law, and in the case of children, guided by the principles of the best interests of children," he said.

Guterres warned that letting this "untenable situation fester" will only fuel "more resentment and despair" and threaten security and stability.

Kurdish authorities in Syria have repeatedly called on countries to repatriate their citizens, but foreign governments have allowed only a trickle to return home, fearing security threats and a domestic political backlash.

IS seized vast areas of Iraq and Syria in 2014 and ruled with brutality until local forces backed by the US-led coalition defeated them, first in Iraq in late 2017 and then in Syria in March 2019.

Guterres has been in Iraq since late Tuesday on a "solidarity" visit, and will travel next to Qatar to attend the summit of the Least Developed Countries which opens on Sunday.