In a statement, Agnes Callamard, a UN special rapporteur on extrajudicial, summary or arbitrary executions, expressed "serious concerns" over the men's fate.

"There are serious allegations that the sentences were handed down following unfair trials, with the accused having no adequate legal representation or effective consular assistance," Callamard said.

She said Iraq's legal system was "marred by very serious structural problems".

Hundreds of foreigners have been sentenced to death or life imprisonment in Iraq for belonging to the Islamic State (IS) jihadist group.

European governments have long debated whether and how to take back former fighters and their supporters amid a host of security, political and legal issues.

The seven French nationals were arrested by the Syrian Democratic Forces (SDF) and subsequently transferred to Iraq in February, allegedly at the request of the French government or with its suspected involvement, Callamard said.

Once in Iraq, they were reportedly subject to torture or other ill-treatment, she added.

"In these circumstances, the transfer of persons to Iraq for prosecution is illegal. I am particularly disturbed by allegations that France may have had a role in this transfer, given the risk involved of torture and unfair trials and that they would likely face the death penalty," Callamard added.

She has written to French Prime Minister Edouard Philippe with her concerns.

Callamard named the men as Fodil Tahar Aouidate, Mourad Delhomme, Karam El Harchaoui, Bilel Kabaoui, Leonard Lopez, Brahim Nejara and Vianney Ouraghi.

Last week, Iraqi Prime Minister Adel Abdel Mahdi declared that his government was not empowered to reduce the sentences of foreign jihadists facing death.

Iraqi law stipulates the death penalty for anyone joining a "terrorist group" -- even those who did not take up arms.

France and other European countries have strongly resisted the repatriation of their nationals suspected of joining IS. But France is also staunchly opposed to capital punishment.