"They’ve made civilians into a human shield, keeping them from leaving the city and using their houses as bases," Ghasri was quoted as saying by The Times on Monday.
According to the spokesman, thousands of civilians were trapped in the Sirte areas seized by the Daesh.
"[Daesh militants] are also using residential roofs as sniper positions, preventing us from moving forward," Ghasri added.
Libya has been in a state of turmoil since its longtime leader Muammar Gaddafi was deposed and killed in 2011. Daesh, which is outlawed in many countries including Russia, seized a pocket of Libyan territory centered in Gaddafi's hometown of Sirte in November 2014.