On Wednesday, Tripoli's forces took Gharyan, south of the capital, which eastern forces loyal to Khalifa Haftar had used as their main supply base to attack Tripoli, home to the recognized administration.
Officials showed journalists weapons they said had been seized, among them sophisticated U.S.-made Javelin anti-tank missiles.
They also presented advanced Chinese-made laser-guided artillery shells, and said combat drones had been also been captured, in addition to some 150 prisoners.
Inscriptions on the Javelin missiles said they had originally belonged to the armed forces of the United Arab Emirates, one of Haftar's main backers.
U.N. reports have previously said that the UAE and Egypt have been arming Haftar's Libya National Army (LNA) since 2014, but details have been unclear.
Oded Berkowitz, Israeli security analyst and deputy chief intelligence officer of the MAX consultancy, said it was the first time that Javelins had been sighted in the Libya conflict.
"The weapons themselves are highly advanced but wouldn't be a game changer in Libya," he said. "The real political game changer is the fact that advanced U.S. systems were delivered to a third party, and this may push the U.S. to oppose the UAE and their support for the LNA."
Haftar began his assault on Tripoli on April 4, surprising the United Nations, which had been preparing for a national conference to try to end the chaos gripping Libya since the fall of Muammar Gaddafi in 2011.
The offensive by the LNA, which is allied to a parallel administration in the east, has not made it beyond the southern suburbs of Tripoli.Turkey has supplied Tripoli's forces with drones and other military equipment, according to diplomats.