Hundreds of captured Islamic State fighters have been imprisoned by Kurdish forces in the parts of northeast Syria that the Kurds control with US support. Thousands more women and children who lived in the caliphate but did not necessarily take part in the fighting are living under tight control inside Kurdish-run camps. Many of them are European citizens.
"There is a problem. We are aware of that in Europe," Luxembourg Foreign Minister Jean Asselborn said. "If we want to find a reasonable solution, then we have to discuss this, not send tweets back and forth. That doesn't make sense."
Among the challenges EU nations face is that it is often difficult to gather evidence of participation in violence by their citizens in the Islamic State, forcing prosecutors to try them on lesser charges that carry penalties of only a few years in prison. Politicians are hesitant to take the risk of one of the returnees plotting an attack.
But some security services believe that it would be safer to have potentially dangerous citizens inside their home countries, where they can more easily be monitored, than to have them float free in the tumult of the Middle East, analysts say.
"You will have a security problem if you leave them out there," said Rik Coolsaet, a terrorism expert at the Egmont Institute, a Brussels-based think tank. "You can follow them here, and you can't follow them there."
France has started planning how to bring back its more than 100 fighters from the camps, along with women and children. Other countries are considering what to do.
The political challenge extends beyond the adults who may still sympathise with the aims of the Islamic State. Many of them started families while living in the caliphate, and Kurdish authorities estimate that at least 1,300 children, many of them younger than 6 years old, are inside their camps. European countries have hesitated about their obligations to those children, and several are debating whether bringing the children home without their parents would be a violation of the children's rights.
In Belgium, the government is appealing a court order to bring back six children ages 7 and under, along with their two Belgian mothers. The appeal is scheduled for Wednesday.
"You cannot say that a child of 8 months is dangerous," said Walter Damen, attorney for the children and their mothers. "You cannot say that a child of 8 months is being indoctrinated by the Islamic State."