Libyan forces battling to oust Islamic State from Sirte on Wednesday captured a large convention hall complex in the city center, seizing a symbolic base where militants once held meetings and flew their black jihadist flag.

Securing the Ouagadougou Conference Centre as well as hospital and university buildings would mark the biggest advance made by Libyan forces in weeks. The United States 10 days ago began air strikes on Sirte, which fighters say hastened their progress.

“Our forces have complete control of the whole of the Ouagadougou complex, they even advanced some distance beyond the complex,” Rida Issa, a spokesman in the forces’ media office, said. Photos posted on the media office’s Facebook page showed fighters with armed pickup trucks and a tank milling around next to the convention hall, which was pockmarked by shelling.

The large domed building is a landmark in Sirte, hometown of late dictator Muammar Gaddafi, and was used for meetings and religious instruction by Islamic State (IS) after they took control of the city last year.