BEIRUT (AP) — A U.S. official says the fight for Raqqa, the Islamic State group’s de facto capital, will “only accelerate” as the militants lose their grip on Iraq’s Mosul.

Brett McGurk, the U.S. envoy for the global coalition against IS, says the U.S. and its local allies, the Kurdish-led Syrian Democratic Forces, are prepared for a protracted fight for the northern Syrian city. He spoke at a press conference in Baghdad on Wednesday.

The jihadist group is locked in twin battles for its key strongholds, Raqqa and Mosul. The SDF launched its attack on Raqqa on Tuesday and have already captured a neighborhood on the city’s eastern end. Iraqi forces have been battling IS in Mosul since October.

McGurk said the fight for Raqqa will be a “difficult and very long-term battle.”