A Libyan official says U.S.-backed forces in Libya have liberated "70 percent" of the city of Sirte, the Islamic State group's last bastion in the North African country.

Mokhtar Khalifa, the Sirte mayor, has told The Associated Press that the city's southern and western sections are under control of the Libyan fighters loyal to the U.N.-brokered government as of Thursday.

He says "the international support has made a big difference" in the battle against IS in Sirte, which has been underway for months.

Since Aug. 1, U.S. warplanes have carried out a series of airstrikes targeting IS positions in Sirte. The Libyan fighters, mainly from the western city of Misrata, captured on Wednesday the city's sprawling convention center known as Ouagadougou, which IS had turned into its headquarters.