US-backed Syrian forces are urging that captured Islamic State militants be returned to their countries of origin, as they push for coalition troops to stay.
Hundreds of foreign jihadist fighters held in Syria represent a "time bomb" and could escape and threaten the West unless countries do more to take them back, the Kurdish-led, US-backed authorities holding them say.
The fate of foreign fighters who joined Islamic State, as well as of their wives and children, has become more pressing in recent days as US-backed fighters plan an assault to capture the last enclave of the group's self-styled Caliphate.
US President Donald Trump said on Sunday European countries must do more to take them back or "we will be forced to release them".
But European countries say there is no simple solution. Fighters must be vetted and prosecuted if they return.
"It is clearly not as easy as what has been put forward in the United States," German Foreign Minister Heiko Maas said on Monday ahead of a Brussels meeting with EU counterparts.
"These people could only then come to Germany if we can ensure they are immediately put in custody. It's not clear to me how all that can be guaranteed."
Abdulkarim Omar, co-chair of foreign relations in the region held by the US-backed Syrian Democratic Forces, said authorities there were holding some 800 foreign fighters.
Around 700 of the fighters' wives and 1500 of their children are also in camps. Dozens more fighters and family members are arriving each day.
"It seems most of the countries have decided that they're done with them, let's leave them here, but this is a very big mistake," Omar said. Their home countries must do more to prosecute foreign fighters and rehabilitate their families, "or else this will be a danger and a time bomb".
"We will not release them. We could never do this," Omar said. But he reiterated warnings that any attack on the region by Turkey, which has vowed to crush the YPG, would spark chaos, allowing the jihadists to escape.
Meanwhile the commander of US-backed forces in Syria called on Monday for about 1000 to 1500 international forces to remain in Syria to help fight IS and expressed hope that the US, in particular, would halt plans for a total pullout.
The remarks by Mazloum Kobani, the commander-in-chief of the SDF, followed talks with senior US generals at an airbase in northeast Syria and offered perhaps the most comprehensive view to date of his requests for an enduring military assistance from the US-led coalition.
"We would like to have air cover, air support and a force on the ground to co-ordinate with us," Kobani told a small group of reporters who travelled with the US military to an airbase at an undisclosed location in northeast Syria.
Kobani said there was discussions about perhaps French and British troops supporting them in Syria. But he stressed he also wanted at least "a partial group of American forces", who now number more than 2000 in Syria, to stay as well.
US Army General Joseph Votel, head of Central Command, said after the talks with Kobani that he was still carrying out Trump's December order for a complete US withdrawal of American forces.
"We certainly understand what they would like us to do, but of course that's not the path we're on at this particular point," Votel told reporters.