Forces loyal to former general Khalifa Haftar and to internationally recognised Prime Minister Abdullah al-Thani have been battling for weeks against Islamists who have taken control of much of Libya's second city, and the capital Tripoli.
The Benghazi Medical Centre said it received the bodies of seven people and 35 wounded in the clashes, while Al-Jala hospital said it took in four bodies and 25 wounded.
Medical sources said the casualties were all from Haftar's ranks and pro-government forces.
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Islamist militias rarely report their casualties.
The fighting raged in southern and central Benghazi, security sources said.
Farther east, in the Islamist stronghold of Derna, five soldiers were killed on Monday when gunmen attacked a checkpoint in the city's western suburb of Dahr el-Ahmar, a military spokesman said.
Earlier this month the Islamists staged a military parade in Derna with tanks and combatants carrying black flags.
Three years after dictator Moamer Kadhafi was toppled and killed in a NATO-backed revolt, Libya is awash with weapons and powerful militias, and run by rival governments and parliaments.