It was not immediately clear what caused the ammunition supply-point blast at the Brak al-Shati air base near the city of Sabha, some 400 miles southeast of Tripoli, the Libya Herald reported.
The blast was followed by a fire.
The facility stored live ammunition and explosives. Such material can catch fire or degrade and become shock-sensitive over time.
"An unknown group tried to infringe on the warehouse causing this unfortunate incident," district Brigadier Mohamed al-Dahabyi told China's official Xinhua News Agency.
The air base's ammunition compound has been hit by a recent rash of raid-like arms and ammunition thefts, the Herald said.
Five days before Thursday's hit, "dozens of gunmen stormed the air base and looted all the ammunition inside the storage, including ammunition for heavy weapons such as rockets and artillery," base commander Col. Ahmed al-Hasnawi told the newspaper.
Libya TV blamed the raid on civilians wanting the ammunition for its valuable copper.
The privately owned Libyan al-Tadamun news agency said more than 30 people were killed in the blast.