Any transfers of suspected foreign jihadists and their relatives out of Syria should be transparent, Human Rights Watch told AFP, as camps in the northeast fill with families of different nationalities.
With the crumbling of the Islamic State group, France is now considering bringing dozens of accused French jihadists, as well as their wives and children, back home from the detention centres and camps run by US-backed forces fighting IS.
"We would definitely like to be present (during the transfer), or at least there should be some transparency," Nadim Houry, HRW's director of counter-terrorism, told AFP in the northern Syrian town of Amuda late Wednesday.
"As we speak, there may already be transfers happening. There's been a total lack of transparency, and bad things happen in the dark," he warned.
Tens of thousands of foreigners are estimated to have joined IS since 2014, but they have streamed out of the jihadists' collapsing "caliphate" in recent years.
The US-backed Syrian Democratic Forces, who are bearing down on the shrinking pocket of IS territory in east Syria, told AFP they were detaining foreign fighters on a "daily basis."
The SDF are also holding hundreds of women and children who were born to alleged IS jihadists, including French nationals, in two main prison camps in the north.
Authorities at one of the camps, Al-Hol, say they have received more than 1,000 foreign nationals since fighting against IS's last positions ramped up in mid-December.
- 'Guantanamo on the Euphrates' -
On Wednesday, dozens of foreign women and their young children, who had recently arrived from the battered IS pocket further south, could be seen waiting in a reception area in Al-Hol.
The women wore black veils covering everything but their blue eyes and called out to their pale, thin children in English and French.
They were waiting to be assigned tents in the cordoned-off section of the camp where foreigners are held, and were not allowed to speak to reporters.
The Kurdish administration in northeastern Syria has spent months calling for the foreigners' countries of origin to take them back.
Those nations are often reluctant, but the issue has taken on greater urgency amid fears of a security vacuum since US President Donald Trump's shock announcement in December that US troops would withdraw.
Washington has also begun pressuring countries to repatriate foreigners in recent days.
French sources have told AFP that an estimated 50 adults and 80 children could be brought back to France, but authorities have not confirmed any planned transfer.
"While this debate is taking place in France, it's not clear it has manifested itself in any concrete measures on the ground," said Houry, whose team plans to visit foreigners in the camps.
HRW is seeking clarity on the numbers that might return, what route they would be transferred through, and whether children would be separated from their parents.
France has a responsibility not to leave its citizens, including children under seven years old, in legal limbo in a "Guantanamo on the Euphrates," Houry said.
"We are confident that once they actually hit France, there is a mechanism in place," he said.
"What we're concerned about is what is going to happen between now and then. We're in a grey zone."