An army commander blamed Islamist fighters for the rocket strike. Fighting for control of the country's second-largest city has been going on for a year, part of a conflict between former rebel groups who helped topple Muammar Gaddafi in 2011 but now back two rival governments.
"The army has made much progress and they suffered big losses," said army commander Fadhal al-Hassi, referring to the Majlis al-Shura, an umbrella group of mostly Islamist factions. "We had been worried they would take revenge for that and indeed now they fired rockets."
Army forces backing the official government, based in the east since losing Tripoli to a rival faction, have expelled the Islamists from several districts but do not control the entire city.
To the east of Benghazi, Islamic State militants attacked a security checkpoint near Derna, an Islamist hotspot, a security official said. Two soldiers were killed and five wounded, he said.