Fires at oil storage tanks in the major Libyan terminal of Ras Lanuf were still raging on Friday more than 24 hours after an attack by Islamic State militants, a firefighting official said.
Mustafa Moussi said fires were burning at five of the terminal's 13 storage tanks, one of which was close to collapse.
"Even with the best efforts of the firefighters we are expecting the worst in the coming hours," he told Reuters. "The scale of the disaster will increase further."
Libya remains plagued by violence and political turmoil almost five years after the uprising that overthrew Muammar Gaddafi. Rival governments backed by armed factions in Tripoli and 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, about 200 km (125 miles) along the coast to the west of Ras Lanuf and the nearby terminal of Es Sider.
This month they have staged several attacks against the 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 (NOC) said up to 3 million barrels of oil could now be lost.
Libya's oil production has dropped to under 400,000 barrels per day, less than a quarter of a 2011 high of 1.6 million bpd, though some oil fields and terminals have been relatively sheltered from the chaos.
A spokesman at the Brega terminal, about 120 km (75 miles) east from Ras Lanuf, said the port was working normally and the Greek tanker Nissos Paros had docked there on Friday to pick up 600,000 barrels of oil.
Separately, a resident and a security source said three Libyan guards working for a Turkish company building a road near the town of Madanah, about 160 km (100 miles) southeast of Ras Lanuf, had been killed by suspected Islamic State militants who kidnapped them last week. A fourth guard had been released, they said.
(By Ayman al-Warfalli; Writing by Aidan Lewis; Editing by Katharine Houreld)