Europe must take in hundreds of ISIL fighters captured in Syria or else the United States may be forced to release them, US President Donald Trump warned.

He made the comments in a series of tweets on Saturday demanding that Britain, France, Germany and other European allies put more than 800 detained cadres from Islamic State of Iraq and the Levant (ISIL, ISIS) with their citizenship on trial.

The president issued the warning as a US-backed militia battles the last remaining ISIL combatants in a tiny sliver of territory in eastern Syria.

"The caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them," Trump said of the war prisoners.

Trump caused widespread concern among US allies in December when he suddenly announced the US would withdraw about 2,000 troops from Syria and that ISIL had already been defeated.

Critics warned the armed group remains a threat and a pullout could lead to its resurgence, with US allies in the region not equipped to handle it alone.

James Jeffrey, the US special representative on Syria, said on Sunday at a security conference in Germany the troop drawdown will not be an "abrupt or a rapid withdrawal".

"It's going to be an orderly, step-by-step withdrawal and at each point … we are looking at our underlying goals. Our general principle is, particularly with ground forces, not to keep them on when other people can be doing the job," said Jeffrey.

Several hundred ISIL soldiers remain in a one-square kilometre area, with the expectation most will fight to the death.

As the SDF advanced under heavy US air attacks in recent days, thousands of civilians have fled the area, along with defeated fighters trying to escape unnoticed.

SDF spokesman Mustafa Bali said on Sunday the fighting for Baghouz continues and he accused ISIL of holding about 1,000 civilians hostage after retreating into tunnels underneath the village. (Aljazeera)

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