Islamic State capital cities of Raqqa and Mosul in Syria and Iraq respectively are close to being choked off after the fall of the last major supply route.

Kurdish fighters backed by U.S. airstrikes and special forces advisers have made major progress in taking back the town of Shaddadi in northeastern Syria, which serves as a supply route to connect the de facto ISIS capital to Mosul, Military.com reports.

Aside from Shaddadi, Kurdish forces this week captured Thawrah, yet another supply route for Raqqa. Now that Shaddadi and Thawrah are in the bag, Syrian Kurdish forces are moving rapidly towards Raqqa to isolate it from further support. However, there are two reasons Raqqa may still remain in ISIS hands. First, there are not many Kurds in Raqqa, which takes away part of the motivation for an assault, and second, the city may be able to survive without access to supply lines.

“Four months ago, we did not have momentum,” Joint Chiefs Chairman Gen. Joseph Dunford said Thursday at a House Committee on Appropriations hearing. “Today, I can tell you with authority we do have momentum. There’s a lot of work left to be done, but the enemy is under great pressure. My assessment is the trajectory is in the right direction.”

But Kurdish forces closing in on Raqqa may not mean the end of ISIS, as militants have aggressively expanded to western Syria and held the city of Palmyra, despite an onslaught of Russian airstrikes. Pentagon estimates, however, point to a much lower fighter presence. The group is reportedly down to just 15,000 fighters. Still, if ISIS holds on to Palmyra and seizes control of the city Homs, it may hold on to Syria for some time.

What seems apparent is that ISIS would rather fight Syrian government forces than Kurdish forces, mostly because the latter is backed by U.S. airstrikes. In Palmyra, Syrian forces, supported by Russian airstrikes, haven’t been able to make a single dent in ISIS defenses.

“ISIS is prioritizing fighting the Syrian regime over Kurdish forces. We are at the beginning of that trend,” Jennifer Cafarella, a fellow at the Institute for the Study of War, told The Daily Beast. “They are ceding territory in the north while consolidating in central Syria and positioning themselves in the west.”

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