Forces of Libyan commander Khalifa Haftar attacked opposition targets in the eastern city of Derna on Tuesday using aircraft and artillery, taking a wheat silo and villages on the outskirts, military officials said.
It was the first major operation since Haftar announced last week that he would seize the last stronghold of opposition to his Libyan National Army (LNA) in the east of the country.
The LNA has surrounded the city, on the coastal highway between Benghazi and Egypt, and has long threatened to begin ground operations there. Its campaign had previously been limited to occasional air strikes and bombardments.
Derna is controlled by a coalition of Islamists and rebel veterans known as the Derna Mujahideen Shura Council (DMSC).
After Libya split between rival camps in the east and west of the country in 2014, Haftar gradually emerged as the dominant figure in the east. He is aligned with a parliament and government based in the east and opposes the internationally recognised government in the capital Tripoli.
The United Nations is leading efforts to stabilize Libya and prepare it for elections before the end of the year.
Egypt, which backs the LNA, has several times carried out air strikes in Derna on what it said were training camps sending militants into Egypt.
Troops were advancing on five frontlines supported by air strikes and artillery, said Abdel-Karim Sabra, a spokesman for the operation.
The troops advanced to villages some 4 kms (two and a half miles) from the city, LNA sources said.
Sabra did not have casualty figures but websites close to the DMSC said five people were killed in air strikes.