With controversy growing in France over the sentences handed out to the nine for joining the Islamic State (IS) extremist group, the 40 lawyers urged Paris to keep the men alive, irrespective of their crimes.

A court sentenced two more French citizens to death on Sunday, meaning that nine French jihadists are now at risk of execution in the country.

"We have taken a historic risk, which, if it is realised, will leave an indelible stain on the mandate of Emmanuel Macron," said the lawyers, who included some of the country's best known legal professionals such as William Bourdon, Henri Leclerc and Vincent Brengarth.

It would mean allowing a "legal assassination which is now proscribed by the majority of countries on the planet with the exception of Saudi Arabia, Iraq, China and the United States," said the open letter, published on the website of radio station Franceinfo.

"It equates to renouncing our engagements by permitting the overseas implementation of the death penalty," they added.

"It would be a great disgrace for our country to make these death sentences possible."

The French citizens were handed over to Iraqi authorities in January by a US-backed force fighting the jihadist group in Syria.

Hearings for the last two suspected French IS members to be tried in Baghdad were set for Monday.

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.

French government spokeswoman Sibeth Ndiaye insisted Sunday that officials were intervening "at the highest level" in the cases.