Libyan forces said they captured a residential neighborhood in central Sirte from Islamic State on Monday, leaving just one district of the city still occupied by the militants.
Spokesman Rida Issa said Sirte's neighborhood Number One was "completely liberated", but added that the brigades, backed by U.S. air strikes, were "in the process of clearing any remnants of Daesh (Islamic State)" from the area.
For more than three months, brigades aligned with Libya's U.N.-backed government have been battling to oust Islamic State from Sirte, a coastal city that the militant group turned into a regional stronghold last year.
The United States began an air campaign over Sirte on Aug. 1, and had by Sunday carried out 92 strikes against Islamic State.
Militants have defended themselves with suicide bombers, snipers and mines, inflicting heavy casualties on brigade fighters. At least six brigade members were killed and 12 wounded on Monday, according to hospital officials.
The latest gains came after the brigades made significant advances on Sunday in which 35 of their men were killed and more than 200 wounded.
Many of the latest casualties were caused by mines, car bombs or artillery fire, according to hospital officials and brigade fighters.
On Monday, the brigades could be seen moving tanks and other vehicles toward forward positions.
There was a pause in fighting last week as the brigades said they were giving a chance to the wives and children of Islamic State fighters to leave the battle zone ahead of a final push.
Most brigade fighters are from the city of Misrata, about 230 km (140 miles) northwest of Sirte. They counter-attacked after Islamic State advanced toward Misrata at the beginning of May.