The militants took control of the al-Qardabiya airbase in Sirte after a local militia tasked with defending the facility withdrew from their positions.
Affiliates of Isis, also known as Islamic State, already control large parts of Sirte, the birthplace of former Libyan leader Muammar Gaddafi and a former stronghold of his supporters.
The city lies halfway between the major city of Benghazi, located in the east of the country, and the capital Tripoli, in its west. It sits on a costal highway that links the east and the west of the country.
A spokesperson for the Islamist 166 Battalion militia told the Associated Press news agency that his force retreated after reinforcements failed to arrive from Tripoli.
The 166 Battalion is affiliated to an Islamist faction currently in control of the country’s capital.
In pictures: Coptic Christians mourn victims of Isis beheadings
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Libya is currently engulfed in a civil war nearly four years after the fall of Colonel Gadhafi, the country’s longstanding dictator.
The country has an elected government and parliament which has relocated to the east of the country in Tobruk and Bayda after Islamists took control of Tripoli.
The Islamic State affiliates have branded the Islamist faction in control of the capital “infidels”, though the local group has so far shied away from armed conflict.
Isis has also sized control of the nearby eastern city of Darnah and a major, important water distribution work.
The group has tried and failed to take oil fields in the region, having been repelled by local fighters.
No such attacks have yet taken place. The group has however abducted and beheaded a number of migrant worker Christians in the country, including those from Ethiopia and Egypt.
Additional reporting by AP
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