An Iraqi court on Sunday sentenced three French citizens to death for joining the Islamic State group, the first IS members from France to be handed capital punishment, a court official said.

Captured in Syria by a US-backed force fighting the jihadists, Kevin Gonot, Leonard Lopez and Salim Machou were transferred to Iraq for trial. They have 30 days to appeal.

Iraq has taken custody of thousands of jihadists repatriated in recent months from neighbouring Syria, where they were caught by the US-backed Syrian Democratic Forces during the battle to destroy the IS 'caliphate'.

The US-backed Syrian Democratic Forces (SDF), who waged a gruelling campaign against the Islamic State group's 'caliphate', have transferred thousands of suspected jihadists to Iraq for trial [file photo]

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

Its courts have condemned many to life in prison and others to death, although no foreign IS members have yet been executed.

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

They have also raised the question of whether suspected IS jihadists should be tried in the region or repatriated to their countries of origin, in the face of strong public opposition.

Those sentenced on Sunday were among 13 French nationals caught in battle-scarred eastern Syria and handed to Iraqi authorities in February on suspicion of being members of IS's feared contingent of foreign fighters.

One was later released as it was found he had travelled to Syria to support the Yazidi religious minority -- the target of a particularly brutal IS campaign that rights groups say may have amounted to genocide.

The remaining 12 were put on trial under Iraq's counterterrorism law, which can dole out the death penalty to anyone found guilty of joining a 'terrorist' group, even if they were not explicitly fighting.

Gonot, who fought for IS before being arrested in Syria with his mother, wife, and half-brother, has also been sentenced in absentia by a French court to nine years in prison, according to French research group the Center for the Analysis of Terrorism.

Machou was a member of the infamous Tariq ibn Ziyad brigade, 'a European foreign terrorist fighter cell' that carried out attacks in Iraq and Syria and planned others in Paris and Brussels, according to US officials.

Lopez, from Paris, travelled with his wife and two children to IS-held Mosul in northern Iraq before entering Syria, French investigators say.

His lawyer, Nabil Boudi, condemned the trial as 'summary justice'.

Kevin Gonot, Leonard Lopez and Salim Machou were transferred to Iraq for trial and now have 30 days to appeal their conviction [file photo]

The French government had 'guaranteed us that French citizens would all be entitled to a fair trial, even in Iraq,' he told AFP.

But Lopez had been sentenced to death 'based solely on a series of interrogations in Baghdad jails', he said.

Iraq declared victory over IS in late 2017 and began trying foreigners accused of joining the jihadists the following year.

Rights groups including Human Rights Watch have criticised Iraq's anti-terror trials, which they say often rely on circumstantial evidence or confessions obtained under torture.

Baghdad has offered to try all foreign fighters in SDF custody -- estimated at around 1,000 -- in exchange for millions of dollars, Iraqi government sources have told AFP.

Among those sentenced to life in prison are 58-year-old Frenchman Lahcen Ammar Gueboudj and two other French nationals.

Iraq has also tried thousands of its own nationals arrested on home soil for joining IS, including women, and begun trial proceedings for nearly 900 Iraqis repatriated from Syria.

The country remains in the top five 'executioner' nations in the world, according to an Amnesty International report in April.

The number of death sentences issued by Iraqi courts more than quadrupled between 2017 and 2018, to at least 271.

But only 52 were actually carried out in 2018, according to Amnesty, compared with 125 the year before.

Analysts have also warned that prisons in Iraq have in the past acted as 'academies' for future jihadists, including IS supremo Abu Bakr al-Baghdadi.