The announcement comes amid an a major offensive by the east-based forces to take Tripoli. The capital city is the seat of a rival Libyan government, which is supported by the United Nations.
Libya is currently governed by dueling authorities in the east and in Tripoli in the west, each relying on different militias.
Commander Khalifa Belqasim Haftar declared a “final” and decisive battle to take Tripoli last month. Fighting and shelling between the two sides has been raging there since April, with neither side making much progress.
Ahmed al-Mosmari, a spokesman for Haftar’s self-styled Libyan National Army, announced Sirte had been taken at a press conference in the eastern city of Benghazi.
Capturing Sirte would be a major win for Haftar’s forces, should they maintain control of the city, which is about 230 miles (370 kilometers) east of Tripoli.
That deal, along with a separate agreement on maritime boundaries between Turkey and Libya, has led to anger across the region and beyond.
Some Sirte residents confirmed that the city was now under the control of LNA fighters. Ahmed Awas, 42, a petroleum engineer who lives in central Sirte, said the sounds of gunfire could be heard in his neighborhood Monday afternoon. He said advancing battalions included fighters from the area.
The latest push for Sirte was over in a matter of hours, according to al-Mosmari. He said that the city was taken Monday in a "lightning" offensive.
Sirte was the hometown of former president Moammar Al-Gadhafi. He was killed in 2011 after an international military coalition helped Libyan rebels oust him from power.
After that, Sirte was controlled for a time by Islamist militants who pledged allegiance to the Islamic State group, until being defeated in 2016.
Libya's east-based government is backed by the United Arab Emirates and Egypt, as well as France and Russia.
The Tripoli-based government receives aid from Turkey, Qatar and Italy.
The fighting threatens to plunge oil-rich Libya into another bout of violence similar in scale to the 2011 conflict.