Four people were killed and 15 wounded when a rocket hit a residential district in the eastern Libyan port city of Benghazi on Sunday, medics said.
There was no immediate claim for the attack. Islamic State militants have claimed previous rocket attacks in Libya's second-largest city where army forces allied to armed locals have been fighting Islamist groups for more than a year.
The rocket attack on the busy Beirut Street came hours after a jet had attacked a bulldozer loaded with ammunition west of Benghazi, according to Nasser al-Hasi, an air force spokesman.
Chaos reigns in Libya where fighting over the port city is part of a wider conflict between former rebel groups who helped topple Muammar Gaddafi in 2011 and now back rival governments.
Both sides command loose coalitions of former anti-Gaddafi rebels calling themselves armies, which split along political, regional and tribal lines after Gaddafi was ousted and killed.
Islamic State militants have exploited the chaos since then and have killed dozens of foreign Christians and attacked oil fields and a luxury hotel in Tripoli.