EU countries have a "short window" to decide whether to repatriate their nationals -- including hundreds of very young children -- held in camps in Syria after the defeat of the Islamic State group, MEPs heard Thursday.

A US troop withdrawal and subsequent Turkish military incursion into northeast Syria last month has injected urgency into the issue, the European Parliament lawmakers were told, in a session on the fate of children of foreign fighters.

Control of the camps is slipping as Kurdish SDF fighters formerly allied to the US become squeezed between the hostile Turkish and Syrian armies, raising the risk of those in the camps escaping, perhaps to Europe in some cases, or falling into the hands of President Bashar al-Assad's forces, they heard.

"There's a short window now, of perhaps one or several months, while the camps with family members of European foreign fighters are still under the control of the SDF," Christiane Hoehn, a senior EU counter-terrorism official told the committee.

The changed battlefield circumstances in Syria "have created new circumstances and this might need to lead to the need to redefine policy," she said.

This is feeding into a long-running debate over what to do with Europeans among the captured Islamic State (IS) group fighters and their families.

There is no common, coordinated EU approach on the issue.

Instead each EU country is trying to balance its legal obligations towards its citizens against the danger posed by bringing hardcore IS militants back home, often without enough admissable evidence to lock them up for any significant period.

Deadly attacks in Europe in the past few years claimed by IS -- including those in Belgium, Britain, France and Germany in 2015 and 2017 that killed more than 290 people -- have reinforced resistance to repatriation.

The al-Hol camp in northeast Syria is the main site holding families of IS foreign fighters
The al-Hol camp in northeast Syria is the main site holding families of IS foreign fighters AFP / Delil SOULEIMAN

But the question of what to do with children of IS fighters, viewed as victims more than as threats, has become sensitive and tricky.

EU countries have already brought back orphans and unaccompanied minors from the camps in Syria. But they are loath to do so for mothers -- some deeply radicalised -- who refuse to relinquish custody of their offspring.

Around 700 to 750 children with European links are being held in the northeast Syria camps, with 300 of them said to be French, a UNHCR human rights official, Marie-Dominique Parent, told the MEPs.

She added that half of the minors of all foreign nationalities -- most of them of Iraqi descent -- in those camps were under age five.

"Other figures seen, not only for the northeast of Syria, include around 200 children from the Netherlands, around 160 from Belgium and 60 from the UK," she said.

"Germany and Sweden are also reported to have significant numbers of detainees but I have not seen the figures of the number of children."

Paul Van Tighelt, head of the threat analysis unit in Belgium's OCAM anti-terrorist agency, noted that the biggest danger in Europe was from extremist "lone actors", of both the religious and the far-right varieties.

The "difficult and delicate" situation of IS-affiliated children, he said, demonstrated "the need for a common European or at least a multilateral approach," and especially systematic information-sharing between states on suspect individuals.