BEIRUT (AP) — U.S.-backed forces in Syria say the battle for the Islamic State group's de-facto capital, Raqqa, has reached its "final stages" with the opening of a new front.

The Syrian Democratic Forces said in a statement Wednesday they have captured the city's grain silos from the militants in a surprise offensive on northern neighborhoods mounted five days ago.

The predominantly-Kurdish SDF say they want their own autonomous zone in a federated Syrian republic. They operate independently of the Damascus-based Syrian government, which is supported by Russia and Iran.

The Britain-based Syrian Observatory for Human Rights says the SDF control 90 percent of Raqqa. It says just 300 to 400 IS militants remain in the city.

IS still controls broad swathes of eastern Syria and pockets of territory in neighboring Iraq.