Though Syria’s Kurdish faction, the YPG, insists there is no imminent plan to attack the ISIS capital of Raqqa, recent gains near the city have fueled a flurry of defensive building by ISIS forces, and questions about whether they are prepared to fight off the Kurds.
Even when the attacks eventually do come, the YPG hopes to sit back in a more supporting role and hand over the task of attacking the major city to smaller secular rebel factions closely allied with the US. Such factions have supported the recent Kurdish gains.
The situation is complicated, with Turkey in particular griping about the growing YPG gains, and particularly the growing refugee crisis the Kurdish gains are causing, as Arab locals are fleeing deeper into ISIS territory, or fleeing into Turkey to escape Kurdish rule.
These smaller secular rebel groups have never been particularly successful on their own, however, even with US backing, and it’s hard to imagine they’d be able to make a serious push against Raqqa without heavy Kurdish support. Even then, this is a big target, and one ISIS will doubtless defend.