Two of them have "alleged that they were tortured or coerced to confess", the New York-based watchdog said in a statement.

"France and other countries should not be outsourcing management of their terrorism suspects to abusive justice systems," said HRW's acting Middle East director, Lama Fakih.

"These countries should not be sitting idly by while their citizens are transferred to a country where their right to a fair trial and protection from torture are undermined."

A Baghdad court on Wednesday sentenced a Frenchman to death for joining IS, bringing to seven the number of French jihadists on death row in Iraq.

Yassin Sakkam was among 12 French citizens transferred to Iraqi authorities in January by a US-backed force which expelled the jihadist group from its last bastion in Syria.

Sakkam's sentence came despite France reiterating its opposition to capital punishment this week.

Iraq has taken custody of thousands of jihadists in recent months after they were captured in neighbouring Syria.

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

France has long insisted its adult citizens captured in Iraq or Syria must face trial before local courts, while stressing 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.