Libyan pro-unity government forces said they seized a key central district of the Islamic State jihadist group's coastal bastion of Sirte on Friday.
Fighters allied to the Government of National Accord took control of a residential area called the "700 housing units" near Ibn Sina hospital and the city's Ouagadougou conference centre, the GNA's forces said on social media.
They said the jihadists were also coming under air strikes.
A field commander, Siraj Daw, said that fighting was getting closer to the Ouagadougou centre which IS has been using as its field headquarters.
"The resistance has been fierce but we have dealt with the (IS) snipers and wiped them out... We've captured many vehicles and weapons," he told reporters.
"They have many dead," he said, without giving figures.
Since May 12, pro-government forces from the west, Libyan naval forces and eastern militias have pushed the jihadists back into a residential zone inside Sirte.
But their early advances slowed when they entered Sirte on June 9 and reached built-up central and northern parts of the city. IS has hit back with suicide car bombs and sniper fire.
The jihadists overran the city, around 450 kilometres (270 miles) east of the capital Tripoli, in June last year.
Pro-government forces retaking Sirte would be a major blow to IS, which has faced a series of setbacks in Syria and Iraq.
Nearly 200 anti-IS fighters have been killed and more than 600 wounded since the start of the offensive, according to medical sources.
Pro-government forces, commanded out of Misrata 190 kilometres to the northwest, are mostly made up of western militias established during the 2011 revolt that overthrew dictator Moamer Kadhafi.
A militia set up to guard the country's main oil facilities has also been advancing on IS from the east.