Yassin Sakkam was among 12 French citizens transferred to Iraqi authorities in January by a US-backed force fighting the jihadist group in Syria.

"I admit to having sworn allegiance" to IS, he told the court, saying he was paid $70 (62 euros) a month.

He added that he regretted his decision to join the group, and asked to be pardoned.

Sakkam, now 29, left France in late 2014 to fight for IS, posting online pictures of himself carrying arms and speaking to multiple media outlets about IS.

He became one of the most notorious jihadists in France, which has been seeking his arrest since 2016.

Kurdish authorities detained him in Syria in 2017.

His brother Karim carried out a suicide attack at the Iraqi-Jordanian border in 2015, according to the French Terrorism Analysis Centre (CAT).

Sakkam's sentence came despite France reiterating its opposition to capital punishment this week amid a series of similar judgments against French citizens handed to Baghdad.

Iraq has taken custody of thousands of jihadists in recent months after they were captured in neighbouring Syria by the US-backed Syrian Democratic Forces (SDF) during the battle to destroy the IS "caliphate".

They include hundreds of foreigners suspected of IS membership, raising the question of whether suspected IS jihadists should be tried in the region or repatriated.

France has long insisted that its adult citizens captured in Iraq or Syria must face trial locally, while reiterating its opposition to capital punishment.

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

Also on Wednesday, an Iraqi court sentenced Tunisian Mohammed Berriri to death for joining IS, after a hearing lasting less than an hour.

Berriri, 24, admitted to joining the group, saying he thought it was "defending the weak", but said he now regretted doing so.

Sakkam and the six other French citizens handed death sentences in recent days have 30 days to appeal.

The remaining five French suspects face trial in the coming days.

The trials have been criticised by human rights groups, which say they often rely on evidence obtained through torture.

In a statement sent to AFP, a group representing the families of French jihadists asked the government in Paris to "do everything possible to stop this fatal chain of death sentences" and to try them "on our soil".

On Tuesday, Foreign Minister Jean-Yves Le Drian said France was stepping up efforts to stop Iraq executing those convicted.