In this April 3, 2018 file, photo, prisoners play volleyball, in a Kurdish-run prison housing former members of the Islamic State group, in Qamishli, north Syria. (AP Photo/Hussein Malla, File)

The US military has long warned that the fight against ISIS is not over. While the terror group has lost the territory it used to call its "caliphate," there are still tens of thousands of ISIS fighters on the loose in both Syria and Iraq, according to Washington.

On top of that, tens of thousands of people formerly living in ISIS-controlled areas in northeastern Syria -- some of them staunch supporters of the group -- have been taken to large camps. TheĀ largest is al-Hol, which has around 50,000 inhabitants.

Kurdish-led groups also hold thousands of ISIS prisoners, while ISIS leader Abu Bakr al-Bagdadi has recently called on his supporters to break the ISIS prisoners out of jail.

So while ISIS is weakened, it is still around, and still dangerous. This was the main reason the US gave for staying in Syria, even after ISIS lost its last enclaves earlier this year. ISIS, the US believes, still has the power and capability to regroup and re-emerge. The Kurds have warned that the chance of ISIS coming back will rise considerably if Turkey moves into northeast Syria.

"We were doing our best to provide the best kind of security in the prisons and in the camps ... [but] with the Turkish invasion ... we are forced to pull out some of our troops from the prisons and from the camps to the border to protect our people," SDF spokesman Mustafa Bali told CNN.

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