The Syrian Democratic Forces have taken hundreds of foreign IS fighters into custody as they bear down on the jihadists' shrinking pocket of territory in eastern Syria.

Since Thursday, they have transferred 280 Iraqi nationals accused of fighting alongside IS to Iraqi authorities.

An Iraqi government source told AFP on Monday that Iraq had also received 14 French nationals from the SDF.

The source could not specify when the transfer took place or whether the suspects would be tried in Iraq or extradited to France, and the French embassy in Baghdad was not immediately available for comment.

Hisham al-Hashemi, an Iraqi expert with intimate knowledge of the issue, said the transfer had taken place in coordination with the US-led coalition fighting IS.

"Those transferred do not include top IS commanders, but they have a trove of information on Daesh's (IS's) structure in Europe, French jihadist networks, and recruitment cells," Hashemi told AFP using an Arabic acronym for IS.

He said the fighters had also battled government troops in Iraq, which could pave the way for their trial in Baghdad instead of their extradition to France.

Iraq has tried hundreds of foreign IS fighters, including some caught in Syria and transferred across the border.

In August, it sentenced a French man and German woman to life in prison and has also handed down death sentences to around 100 foreigners convicted of IS membership.

French sources have told AFP that an estimated 50 adults and 80 children could be brought back to France from Syria, but authorities have not confirmed any planned transfer.

Earlier this month, Human Rights Watch warned against any secret repatriations.

"There's been a total lack of transparency, and bad things happen in the dark," HRW's head of counter-terrorism Nadim Houry told AFP.