Libya's National Oil Corporation (NOC) has revealed what it calls "catastrophic damage" to the Ras Lanuf tanker terminal after an attack by armed militia.
It confirmed the loss of storage tanks two and 12 at the port, cutting crude oil capacity by 400,000 barrels to 550,000 barrels.
Prior to the attack by forces controlled by Ibrahim Jadhran, Ras Lanuf operated five crude oil storage tanks, it said.
NOC warned of an environmental disaster, with tank two in danger of leaking on Monday, and spreading the blaze to reservoirs one, six and three.
The oil company also said one of its employees was shot in the fighting.
"The NOC board wish him a speedy recovery," it added.
"A number of other employees were robbed by a number of African armed mercenaries fighting alongside the Jadhran militia. Thankfully all NOC employees were evacuated to safety."
NOC has again called for the immediate withdrawal of "Ibrahim Jhadran and his gangs" from the port, the cessation of military operations and the provision of support and assistance to firefighting teams trying to reach the tanks still ablaze.
"This incident will result in the loss of hundreds of millions of dollars in construction costs, and billions in lost sales opportunities," it said.
"Rebuilding the tanks may take years, especially in current security circumstances. NOC applauds the heroes whom risking their lives to save the livelihood of the Libyan people."
Minerva Marine's 104,000-dwt aframax Minerva Lisa (built 2004), which was due to arrive at the port of Sidra to load a crude cargo on Thursday, was advised to stay outside the port, a source told Reuters.
The ship, chartered by trader Petraco, was seen turning away from the port on Thursday morning without loading, according to Reuters ship tracking.
A second tanker, Thenamaris' 105,000-dwt Seascout (built 2004), had been due to arrive at the port on Monday.
