Libyan forces loyal to eastern-based commander Khalifa Haftar said on Monday they had taken control of the strategic coastal city of Sirte in a rapid advance preceded by airstrikes.
Holding Sirte would be an important 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 US airstrikes in late 2016.
Haftar's self-styled Libyan National Army (LNA) said it had taken areas surrounding Sirte, including al-Qardabiya airbase, before moving towards the city centre.
"The commander-in-chief decided on a well-planned, pre-emptive 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 airstrikes.
An LNA military source said forces from the city of Misrata to the northwest, had retreated. Misrata led the campaign against Daesh and is a key source of military power for the GNA.
Earlier, a resident in Sirte 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.
The LNA advance comes as Turkey prepares to send military advisors and experts to Libya to help shore up the GNA, part of rising international involvement in Libya's conflict.
Haftar's LNA has received material and military support from countries including the United Arab Emirates, Jordan and Egypt, according to UN experts and diplomats.