Fires in the major Libyan oil terminal of Ras Lanuf were still raging on Saturday, two days after an attack by Islamic State militants.
Libya remains plagued by violence and political turmoil almost five years after the uprising that overthrew Muammar al Gaddafi.
Rival governments backed by armed factions in the capital, Tripoli, and in the east have battled for power -- and a share of the country's oil wealth.
Islamic State militants have taken advantage of a security vacuum to establish themselves in the city of Sirte.
This month, they staged several attacks against Libya's ports, which have a combined export capacity of 600,000 barrels per day.
They were processing about half of that before both were closed amid fighting in December 2014.
At least seven storage tanks had already been damaged by fires earlier this month.
After Thursday's attack, a spokesman for the National Oil Corporation said up to 3 million barrels of oil could now be lost.