A truck used by Islamic State fighters is seen during a battle between Libyan forces and IS in Sirte, Libya, August 13, 2016. Picture taken August 13, 2016. REUTERS/Stringer

TRIPOLI (Reuters) - Libyan forces fighting to oust Islamic State from their former North African stronghold of Sirte said they had made advances on Sunday in a coastal neighborhood after staging attacks by land, sea and air.

The forces, led by brigades from the city of Misrata and aligned with the U.N.-backed government in Tripoli, secured several key sites in Sirte this week, their progress helped by U.S. air strikes against Islamic State that began on Aug. 1.

Islamic State militants are now encircled in Sirte’s central residential areas.

“The forces have made more progress in neighborhood Number Two, with help from artillery and from naval forces,” spokesman Rida Issa said. Libyan jets carried out two air strikes, he said, and a picture posted by the forces on social media showed guns mounted on a tug boat involved in the operation.

At least four members of the Misrata-led brigades were killed in Sunday’s clashes and 32 were wounded, said Akram Gliwan, a spokesman for Misrata Central Hospital.

The brigades said on Saturday that they had gained control of a radio building previously used by Islamic State for broadcasting. The building is close to the Ouagadougou convention complex, a symbolic landmark in Sirte that the brigades seized this past week, raising the Libyan flag there in place of Islamic State’s black banner.

As of Thursday, the United States had carried out 41 strikes targeting Islamic State fighting positions, vehicles and weapons in Sirte.

Islamic State took over Sirte last year, enforcing its ultra-hardline rule on the city and basing many foreign fighters there. The government-backed brigades launched their campaign to capture Sirte in May, counter-attacking after Islamic State advanced up the coast toward Misrata.