U.S.-backed Libyan militias say they have taken over the Islamic State group's headquarters in Sirte, the militants' final bastion in Libya.

The fighters said on Wednesday that they had seized control of the sprawling convention center that was used as IS's headquarters in the coastal city. The militia fighters, who are mainly from the nearby city of Misrata, launched their offensive against IS in June.

A statement on the forces' Facebook page declares that "Sirte is returning to Libya."

U.S. warplanes have launched a series of airstrikes targeting IS positions in the city. The air support came in response to a request for assistance from Libya's U.N.-brokered government after battles in Sirte stalled.

The militants seized control of the city, the hometown of Libya's former dictator Muammar Gadhafi, in 2015.