The Libyan National Army (LNA), commanded by Khalifa Haftar, announced on Monday that it had captured the strategic coastal city of Sirte in a rapid advance preceded by air strikes.
Holding Sirte would be an important gain for Haftar, who since April has been waging a military offensive on the capital, Tripoli, home to the Government of National Accord (GNA).
Sirte lies in the center of Libya's Mediterranean coast, and has been controlled by GNA-aligned forces since they ejected ISIS from the city with the help of US air strikes in late 2016.
The LNA said it had taken areas surrounding Sirte including al-Qardabiya air base, before moving towards the city center.
"The commander-in-chief decided on a well-planned, preemptive strike and within less than three hours we were in the heart of Sirte," said LNA spokesman Ahmed al-Mismari.
"It was a sudden, swift operation," he said, adding that the advance had been preceded by several hours of air strikes. "Sirte has been totally liberated... from terrorist groups.”
An LNA military source said forces from the city of Misrata had retreated. Misrata led the campaign against ISIS and is a key source of military power for the GNA.
Earlier, a resident in Sirte city center 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.
The LNA advance comes as Turkey prepares to send military advisors and experts to Libya to help shore up the GNA, part of a trend of rising international involvement in Libya's conflict.
In recent weeks there has been an escalation of fighting, shelling and air strikes around Tripoli. On Saturday evening a military college in the capital was hit, killing at least 30 people, a day after the only functioning airport in Tripoli faced its latest closure due to shelling and rocket fire.
