Khalifa Haftar's Libyan National Army said on Monday they had advanced into positions around the city of Sirte in a bid to take control of it.
Capturing Sirte would be a important strategic gain for Haftar, who since April has been waging a military offensive on the capital, Tripoli, home to Libya's internationally recognised Government of National Accord (GNA).
Sirte lies in the centre of Libya's Mediterranean coast, and has been controlled by GNA-aligned forces since they ejected Islamic State from the city with the help of U.S. air strikes in late 2016.
Haftar's Libyan National Army said they had taken areas surrounding Sirte including al-Qardabiya air base.
"The army forces control all the districts surrounding the city of Sirte, and we are heading towards the heart of the city to complete its liberation," said Khaled al-Mahjoub, an LNA spokesman.
A resident in the city centre told Reuters by phone: "We can see convoys of LNA inside Sirte city ... they control large parts of the city now. We also hear gunfire."
There was no immediate comment from GNA forces.