East Libyan forces carried out air strikes around major oil ports on Saturday as they sought to regain control of the area from a rival faction, a military spokesman said.
The eastern-based Libyan National Army (LNA) and allied forces retreated on Friday from Es Sider and Ras Lanuf, two of Libya's largest export terminals, as a faction known as the Benghazi Defence Brigades (BDB) attacked.
The prospect of a renewed escalation of violence around the ports puts at risk the big rise in oil production achieved after the LNA took over four ports in September.
Libya's oil production has been fluctuating around 700,000 barrels per day (bpd), more than double its output last year but well under the 1.6 million bpd the OPEC member was producing before a 2011 uprising.
Though Es Sider and Ras Lanuf have been reopened, they were badly damaged and are operating well below capacity.
It was not clear if the BDB had gained control over them.
"The ports are closed and most of the engineers returned to their homes," a port official in Ras Lanuf told Reuters. "According to workers who left yesterday (the BDB) entered with their vehicles and stationed themselves in front of the ports of Ras Lanuf and Es Sider."
The head of the National Oil Corporation, Mustafa Sanalla, told Reuters the number of workers at the two ports had been reduced and that the NOC and its partners were continuing to monitor the situation.
Officials connected to a self-declared government in Tripoli that opposes the LNA broke into the NOC headquarters on Saturday to make a statement on the oil port attack.
Sanalla said they had "gained access to our premises by force", condemning what he called "cheap tricks ... that try to drag NOC into politics".
"The National Oil Corporation and its facilities should not become a bargaining chip in the country's political conflicts," he said.
The corporation said it met on Friday to look at ways to protect oil facilities in the area and to review loading schedules.
There are no tankers known to be trying to dock at the ports, but shipping sources said one is due to arrive at Es Sider on March 7 to load 630,000 barrels for Austria
's OMV .
In response to the BDB advance, air strikes were carried out in Ras Lanuf, Es Sidra, Ben Jawad and Harawa overnight and into Saturday, LNA spokesman Ahmed al-Mismari told Reuters.
A resident in Ras Lanuf said they heard war planes over the town at dawn on Saturday, followed by explosions, and that strikes continued during the day. Residents posted pictures of fires and damage to buildings apparently caused by the strikes.
The BDB are composed partly of fighters who were ousted by the LNA in Benghazi, where LNA commander Khalifa Haftar
has been waging a military campaign for nearly three years against Islamists and other opponents.
That battle is linked to a wider conflict between political and armed factions based in eastern and western Libya that has severely disrupted oil production for several years. (Additional reporting by Ahmed Elumami in Tripoli and Aidan Lewis in Tunis; Writing by Aidan Lewis; Editing by Clelia Oziel and Hugh Lawson)