Libya’s UN-backed government says 21 people have been killed and 27 wounded in fighting near the capital, Tripoli.
Among the dead is a Red Crescent doctor.
US Secretary of State Mike Pompeo has called for an immediate halt to the fighting.
Rebel forces under General Khalifa Haftar have advanced from the east with the aim of taking Tripoli.
Prime Minister Fayez al-Serraj has accused him of attempting a coup and says rebels will be met with force.