Clashes erupted Sunday night and continued into Monday morning, keeping shops closed and prompting teachers to call around, telling parents to keep their children home from school, the Libyan Herald reported.
"This is really the heaviest fighting I have heard in Benghazi since the revolution," a Benghazi resident told the Herald. "Ansar al-Sharia versus the army, this has been a long way coming."
One senior Libyan military official, speaking on condition of anonymity, told CNN the fighting wasn't planned, but the army was acting in reaction to an attack by Ansar al-Sharia.
There was some confusion as to what prompted the fighting, the Herald said, though it may have been prompted by a checkpoint Ansar al-Sharia set up in the city's Birkah district.
Deputy Prime Minister and Acting Interior Minister Sadiq Abdulrahman said fighting had died down Monday afternoon and Ansar al-Sharia had all but disappeared.
Among the dead were at least seven Libyan soldiers, the Herald reported.
U.S. officials suspected Ansar al-Sharia in the attack on the U.S. Consulate last year that killed four U.S. citizens, including Ambassador Christopher Stevens.