Forces loyal to Libya's unity government fought deadly street battles with the Islamic State group in Sirte, as they pressed an offensive to capture the jihadists' coastal bastion.
Clashes raged around the Ouagadougou conference centre, a sprawling complex that once hosted international summits in the era of ousted dictator Moamer Kadhafi which is now used as an IS command centre.
The loss of Sirte, Kadhafi's home town, would be a major blow to IS at a time when it is under mounting pressure in Syria and Iraq.
From early in the day, forces aligned with the UN-backed Government of National Accord (GNA) pounded IS positions around the complex with heavy artillery fire.
Warplanes also carried out air strikes around the centre and other IS positions inside Sirte, according to social media accounts belonging to the anti-jihadist operation.
Two GNA fighters were killed and eight wounded and taken to a Misrata hospital, further east, the government said.
An AFP correspondent at the scene reported heavy street fighting about two kilometres (one mile) from the Ouagadougou centre.
GNA forces used tanks, rocket launchers and artillery, the correspondent said, while the jihadists responded with machineguns, mortar rounds and sniper fire.
"It was a war with planes and artillery, but now it is street fighting," said one GNA combatant, who declined to be named.
"We are fighting between houses, on the streets, and we won't back down before we eliminate them."
Power struggles have prevented Libya's fledgling government and its allies from ousting IS from its Gulf of Sidra safe-haven for a year.
The GNA, established in Tripoli more than two months ago, has been trying to unify violence-ridden Libya and exert its control over the entire North African country.
Foreign intelligence services estimate IS has 5,000 fighters in Libya, but its strength inside Sirte and the number of civilians living in the city are unclear.
- 'Crack pretty quickly'-
The GNA forces apparently pushed their way from the west into the city centre, and the AFP correspondent saw dozens of 4X4 vehicles deployed along the way.
A Libyan government official said "the battle wasn't as difficult as we thought it would be".
"Their force has dispersed," said the official. "They (Misrata forces) have taken almost all the city now."
The GNA on Thursday said it expected to announce the liberation of Sirte in "two or three days," after its forces thrust into the city centre.
"We're encouraged by the progress they're making," said US special envoy Brett McGurk.
"Once you have a credible force on the ground that moves against them, there is a chance that they could crack pretty quickly."
GNA forces launched the Sirte offensive in mid-May and have seized towns, a power plant and army barracks before Thursday's advance on the city centre.
But analysts have warned that recapturing Sirte would not end IS violence in Libya.
"What will happen to all the forces mobilised against IS?" asked Mohamed Eljarh of the Rafik Hariri Center for the Middle East.
"And Haftar's forces? There is a risk that they turn against each other," he said.
Controversial General Khalifa Haftar is an opponent of the GNA who heads forces loyal to a rival government.
GNA troops are mostly made up of militias from western cities, notably Misrata, and the guards of oil installations that IS has repeatedly tried to seize.
On the GNA's Facebook page, prime minister-designate Fayez al-Sarraj called Friday on "all military forces to unite in the face of our common enemy... and to join the victorious forces".
But Ahmed al-Mesmari, a spokesman for Haftar's forces, told AFP that "the groups that are fighting IS in Sirte are illegitimate militias, loyal to an illegitimate government".
Haftar's forces have reportedly stopped in villages south of Sirte and not advanced on the city.