Libyan forces loyal to eastern-based commander Khalifa Haftar said on Monday they had entered Sirte and had taken control of large parts of the coastal city.
Capturing Sirte would be an 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 US air strikes in late 2016.
Haftar's self-styled Libyan National Army (LNA) said they had taken areas surrounding Sirte including al-Qardabiya air base, before moving towards the city centre.
"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.
An LNA military source said forces from the city of Misrata had retreated. Misrata led the campaign against Islamic State and is a key source of military power for the GNA.
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 a trend 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 UN experts and diplomats.
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.
Sirte, the birthplace of former Libyan leader Muammar Gaddafi, was taken over by Islamic State in early 2015, becoming the jihadist group's most important base outside the Middle East.
The city is just to the west of Libya's oil crescent, a strip of coastline along which several key oil export terminals are located. Haftar's forces seized the oil ports in 2016. 
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