Forces of Libyan strongman Khalifa Haftar on Monday said they had taken control of the coastal city of Sirte from factions loyal to the Tripoli government.
Sirte, some 450 kilometres (280 miles) east of the capital Tripoli, had been held by forces allied with the UN-recognised Government of National Accord (GNA) since 2016.
But on Monday, a spokesman for the rival faction, Haftar's Libyan National Army (LNA), said the city had fallen to his fighters.
"Sirte has been totally liberated... from terrorist groups," Ahmad al-Mesmari, spokesman for Haftar's forces, said on television following an operation that lasted just hours.
The oil-rich North African country has been plunged into chaos since the 2011 NATO-backed uprising that killed longstanding dictator Moamer Kadhafi.
It is now divided between the GNA based in the capital Tripoli in the west and Haftar's forces which are based in the east and also control most of the country's south.
Haftar's LNA earlier Monday said on social media they were "steadily advancing towards the heart of Sirte" after seizing control of Ghardabiya airport on the city's outskirts.
The statement said the operation aimed to "purge Sirte of armed militias".
The airport, located in an airforce base, fell into LNA hands after "forces tasked with protecting it surrendered completely with their equipment" to Haftar's self-styled army, the statement added.
"Clashes are underway" in the southern part of Sirte and in its coastal area, the statement said, adding that some enemy fighters had fled while others were captured by the advancing LNA forces.
Forces loyal to the UN-recognised GNA confirmed they had come under attack in Sirte.
"We are dealing with Haftar's terrorist militias," they said in a statement on Facebook, adding that "mercenaries" from Chad were fighting alongside Haftar's forces.
The UN-recognised government has recently sought help from its ally Turkey.
Last week the Turkish parliament passed a bill approving a military deployment to Libya to shore up the GNA.
On Sunday Turkish President Tayyip Recep Erdogan said soldiers had begun deploying in Libya.
Tensions escalated in Libya when Haftar launched an operation in January 2019 to "purge" southern Libya "of terrorist groups and criminals" and seized several towns after security support from local tribes.
Haftar then set his eyes on Tripoli, launching an offensive on the capital in April to unseat the GNA.
Sirte was at one stage a bastion of the Islamic State group which moved into Libya amid the chaos that followed Kadhafi's ouster.
IS controlled Sirte from early 2015 but the GNA, backed by US airstrikes, evicted the jihadists in a seven-month battle that cost hundreds of lives.