Medics, revising an earlier toll, said at least 25 people had been killed in 24 hours of fighting since early Wednesday, when Khalifa Haftar's forces launched what he called an operation to "liberate" Benghazi.
Tanks spearheaded attacks on an Islamist militia, the "February 17 Martyrs Brigade," while warplanes carried out raids on its headquarters in a western suburb of the Mediterranean city.
The army and Haftar's forces said they captured the February 17 headquarters after several hours of clashes yesterday.
Gunfire and explosions echoed throughout the night, AFP correspondents said.
After a morning lull, fierce clashes broke out today in the western suburbs of Guewarsha and Garyunes, with street fighting near residential areas, witnesses said.
Islamist militias have seized control of large parts of turmoil-gripped Libya since a 2011 uprising against long-time leader Moamer Kadhafi, with the authorities struggling to control them.
The army has this time publicly thrown its weight behind Haftar, who launched a first, unsuccessful, campaign against the Islamists in May, dubbing it "Operation Dignity".
The Islamists include the Ansar al-Sharia jihadist militia, which the United States has branded a "terrorist" organisation.
Since launching Operation Dignity, Haftar and his forces were steadily beaten back to a final redoubt at Benghazi's airport, which has come under attack by Islamists since mid-September.
Last week, more than 50 people were killed in fighting between the rival sides, according to military and hospital sources.
In May, the authorities accused the former Kadhafi-era general -- who spent years in exile before returning to join the 2011 revolution -- of trying to mount a coup.