BEIRUT/BRUSSELS (Reuters) - 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, U.S.-backed authorities holding them said on Monday.

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 U.S.-backed fighters plan an assault to capture the last enclave of the group’s self-styled Caliphate.

U.S. 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 U.S.-backed Syrian Democratic Forces, said authorities there were holding some 800 foreign fighters. Around 700 of the fighters’ wives and 1,500 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”.

European officials complain that dealing with the fate of the detainees has been made more complicated by Trump’s abrupt announcement in December that he plans to pull out the 2,000 U.S. troops protecting the area where they are being held.

“At this stage France is not responding to (Trump’s) demands,” French Justice Minister Nicole Belloubet told France 2 television. “There is a new geo-political context, with the U.S. withdrawal. For the time being we are not changing our policy.”

Omar, the official in the Kurdish-led Syrian region holding the prisoners, said the authorities would never release the fighters, but they could escape in future, especially if the area comes under attack. The Kurdish-led forces are worried about a potential attack from Turkey once U.S. troops leave.

FILE PHOTO - A member loyal to the Islamic State in Iraq and the Levant (ISIL) waves an ISIL flag in Raqqa, Syria June 29, 2014. REUTERS/Stringer/File Photo

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With U.S. help, the Kurdish-led militia are poised to seize Islamic State’s last holdout in eastern Syria. At the height of its power four years ago, Islamic State held about a third of both Iraq and Syria in a self-proclaimed Caliphate.

Bringing militants and their families back home is deeply unpopular in European countries, many of which have suffered militant attack in recent years. European countries say their diplomats cannot operate in an area where Syrian Kurdish control is not internationally recognized.

Pleas from women to return with their children -- such as Shamima Begum, a pregnant 19-year-old who left London as a schoolgirl to become an Islamic State bride -- have stirred up debate in their home countries. Omar said the Kurdish authorities had not been contacted by Britain over her case.

European security services worry returnees will prove a burden on state resources and may radicalize others.

“We have all done everything in our power to lock these people up. This announcement from Trump, I cannot comprehend,” Austria’s Foreign Minister Karin Kneissel said in Brussels.

Belgium’s justice minister called on Sunday for a EU-wide approach to the issue, pointing to doubts the Kurds will be able to maintain control over their territory without U.S. support.

“The Kurds ... could be attacked by the Turks,” Justice Minister Koen Geens told public broadcaster VRT on Sunday. “If the IS fighters are released then we do not know what will happen with them. Control is better than total freedom.”

Several countries are already working quietly to repatriate minors on a case-by-case basis.

Of more than 5,000 Europeans -- most from Britain, France, Germany and Belgium -- who went to fight in Syria and Iraq, some 1,500 have returned, according to police agency Europol.