Mustapha Merzoughi, 37, was sentenced to death by hanging, according to an AFP journalist at the court.

"The evidence and the confession show that you joined the Islamic State group, that you worked in its military branch," the judge told Merzoughi before handing down his sentence.

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

The Iraqi judiciary said earlier in May that it had tried and sentenced more than 500 suspected foreign members of IS since the start of 2018.

"I'm not guilty of crimes and killings. I'm guilty of going (to Syria)," Merzoughi, wearing a yellow prison uniform, told the judge.

"I ask for forgiveness from the people of Iraq, Syria, France and the families of the victims."

Merzoughi told investigators he had served in the French army from 2000 to 2010, including a tour in Afghanistan in 2009.

In France, he lived in the southwestern city of Toulouse, the hometown of jihadist brothers Fabien and Jean-Michel Clain who claimed the deadly 2015 attacks in Paris and were killed fighting in Syria.

Passing through Belgium and then Morocco, the French citizen of Tunisian origin underwent "religious and military training in Aleppo", in northern Syria.

Fellow French jihadist Fodhil Tahar Aouidate also appeared before the court on Monday, but his trial was postponed until June 2 to allow for a medical examination after he alleged he had been beaten in custody.

Aouidate, 32, made a first trip to Syria in 2013 and returned in 2014 to join IS, according to the French judiciary.

After 130 people were killed in IS's 2015 Paris attacks, Aouidate appeared in a video saying it was his "great pleasure and joy to see these unbelievers suffer as we suffer here."

Their trial came a day after an Iraqi court sentenced three French citizens to death for joining IS, making them the first French jihadists to be handed capital punishment.

Captured in Syria by the SDF, Kevin Gonot, Leonard Lopez and Salim Machou were transferred to Iraq for trial. They have 30 days to appeal.