European leaders expressed scepticism Monday about their willingness to cooperate with a request by President Trump to bring home citizens who went to fight with the Islamic State, underlining a security dilemma as the US military prepares to pull out of Syria following the collapse of the caliphate.

Many European nations have been content to leave citizens who may sympathise with the Islamic State in Syria, gambling that their societies will be safer if radicalised citizens are kept far from their borders. But the Kurdish fighters who have kept many of the former caliphate residents under lock and key worry that with the US pullout, they may need to shift resources elsewhere, disbanding camps and allowing the residents to disburse.

Trump over the weekend threatened EU allies on Twitter that if they did not repatriate their citizens, the United States would simply let them go, warning that Europe could face a surge in terrorist attacks as a result.

But his tactic sparked anger at a Monday gathering of EU foreign ministers, where leaders said that they would make no plans under threat from Washington and that counterterrorism policy shouldn't be made by tweet.

Advertisement

"It is surely not as easy as imagined in America," said German Foreign Minister Heiko Maas, who said Germany is discussing the issue with France, Britain and other European countries. The US request is "difficult to implement" right now, he said, because Germany cannot yet guarantee that all returning fighters would be taken into custody immediately while cases were prepared against them.

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."