"For five years, we have been supporting these families who face oblivion and silence on the part of the authorities," Guillaume Martine, a lawyer representing the families, told AFP.

He said his clients had filed a petition with the United Nations Committee Against Torture on Thursday asking it to take up the case of their sons, and to order France to swiftly repatriate them.

The committee, composed of 10 independent experts, issues opinions and recommendations that are not enforceable but carry reputational weight.

The petition was filed on behalf of Sofiane Derrou, Mohamadi Reda, Haroune Belfilali and a fourth man whose name was not given, who had gone to parts of Syria controlled by the Islamic State group between 2014 and 2016.

IS seized control of large swathes of Syria and neighbouring Iraq in 2014, before Syrian forces spearheaded by Kurds and backed by a US-led coalition ousted them from their last patch of land in eastern Syria in 2019.

Kurdish autonomous authorities in northeast Syria have since then been holding around 56,000 people, including 30,000 children, in detention centres and camps.

Among them are IS fighters and their families, as well as displaced people who fled the fighting.

According to the latest information received by their families or reported by the media, the four men are being held in camps and prisons in Syria and Iraq.

Thursday's petition suggests they are being detained without access to medical care and are "subjected to violence daily".

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.

The Committee Against Torture and other UN expert panels have already called on France to repatriate women and children held in the Syrian camps.

But no international body has yet to rule on countries' obligations when it comes to the men stuck in similar conditions, Martine said.

"With this petition, we wish to hold the French state accountable for the fate of its own nationals, whom it abandons to illegal detention, exposing them to undignified conditions and even torture," Martine said.