President Trump told Turkey's president Monday he agreed to the creation of a "security zone" in northern Syria, home to U.S.-backed Kurdish fighters who have beaten back ISIS insurgents. But now those allies face a new enemy if the U.S. pulls out.

CBS News traveled through a part of northern Syria which is under the control of U.S. allies on the ground. But ISIS used to hold large stretches of territory here, and it could be a contested area when U.S. forces pull out.

Those allies are the Syrian Democratic Forces (SDF), a Kurdish-led militia whose partnership with U.S. troops has enraged Turkey, which accuses the force of having links to terrorists on Turkish soil. The Turks have vowed to step up attacks against the SDF, while Mr. Trump has threatened to devastate Turkey economically if it does.

SDF spokesman Nuri Mahmud said ISIS still exists, so allies should work even harder to eliminate it, calling the terror group not just a military force but an extremist mentality.

The American force of roughly 2,000 troops has not only supported local fighters on the ground with equipment and weapons. Crucially, it directs the might of U.S. air power against ISIS targets. Jets provide vital aerial surveillance and wave after wave of devastating airstrikes.

Forces are still on the offensive against ISIS holdouts and they're facing fierce resistance. One senior security source told CBS News he expects the fighting to continue for another two months.