At least nine of the Misrata-led forces' fighters were killed on Thursday and more than 40 wounded, Misrata hospital spokesman Akram Gliwan said.
Libyan forces said they foiled three attempted car bombings in renewed heavy fighting against Islamic State holdouts in the group's former stronghold of Sirte on Thursday.
Forces led by brigades from the western city of Misrata and supported since Aug. 1 by U.S. air strikes have surrounded militants in a gradually shrinking residential area in central Sirte after a four-month campaign.
At least nine of the Misrata-led forces' fighters were killed on Thursday and more than 40 wounded, Misrata hospital spokesman Akram Gliwan said. The forces said they had recovered the bodies of 10 Islamic State militants.
Tanks positioned around the area still occupied by Islamic State launched a continuous barrage of fire, backed up by mortars and anti-aircraft guns, a Reuters witness said. Islamic State fighters responded with sniper fire and rocket-propelled grenades.
A spokesman for the Misrata-led forces, Rida Issa, said groups of militants had been cut off from each other during Thursday's fighting. "Now the Daesh (Islamic State) militants are separated and trapped by our forces in two different areas," he said.
Two of the cars primed to be used as bombs were hit from the ground before they could reach their targets and the third was hit by an air strike, Issa said, though it was unclear whether the strike was from a U.S. or a Libyan plane.
After advancing rapidly on Sirte in May, the Libyan forces have suffered high numbers of casualties from sniper fire, suicide bombings and improvised explosive devices. More than 500 have been killed and more than 2,000 wounded.
The forces are supported by a U.N.-backed government in Tripoli but complain that it has been slow to provide help. Fighters are mostly using outdated weapons and ammunition and lack armoured vehicles and protective clothing.
Islamic State expanded into Libya in 2014, taking advantage of the political chaos and security vacuum that developed after Muammar Gaddafi was toppled in an uprising five years ago.
The jihadist group took full control of Sirte last year, setting up an important base for foreign fighters and expanding its control along about 250 km (155 miles) of Libya's Mediterranean coastline.