An armed faction overran one of Libya's main oil terminals and a nearby airport on Friday, engaging forces loyal to military strongman Khalifa Haftar.
The developments come at a time when Libya is seeking to boost domestic oil production.
The Libyan National Army (LNA), under Haftar's control, took control of the terminals of Es Sider and Ras Lanuf - which have a potential combined capacity of around 600,000 barrels per day - in September 2016.
However on Friday port engineers and a Libyan oil source told Reuters that forces from the Benghazi Defence Brigades - an Islamist group composed partly of forces ousted from Benghazi by the LNA - had taken control of Es Sider port.
Meanwhile videos emerged showing the group's fighters in the nearby Ras Lanuf airport.
The LNA said early on Friday that it had launched airstrikes and ground troops in order to combat attacks by the Benghazi Defence Brigades.
A local medical source who spoke to Reuters said that at least three LNA troops were killed and a further five injured in fighting.
Images of Benghazi Defence Brigades members in Ras Lanuf's airport were
distributed by Libyan media channels on Friday [Twitter]
Local sources who spoke with The New Arab's Libya correspondent Abdullah Sharif also said that fighting had taken place on Friday in the towns of Ben Jawad and Nofaliya, located close to the vital oil facilities.
Late on Friday it remained unclear who controlled the ports, seen as crucial to Libya's hopes of reviving its lucrative, but flagging oil sector.
Es Sider and Ras Lanuf were reopened by Libya's National Oil Corporation seven months ago after the LNA took control of the terminals, enabling a boost in domestic oil production.
While Libya's Tripoli-based and UN-backed Government of National Accord (GNA) condemned escalations of violence in the area on Friday, it has also faced accusations from some in the east of the country of backing the Benghazi Defence Brigades.
The GNA is at odds with Haftar and the LNA who are seen as the figurehead and muscle respectively of a rival government structure based in Tobruk.
Some in Libya praised attacks mounted by the Benghazi Defence Brigades who in part claim that their efforts are aimed at facilitating the return of internally displaced people to Benghazi.
These include Libya's Grand Mufti Sheikh Sadiq al-Gharyani, according to reports in the Libya Observer.
Libya - which has the largest oil reserves in Africa - has more than doubled its oil production from last year up to around 700,000 barrels per day (bdp). However, this figure remains far below the 1.6 million bdp the OPEC member was pumping before a NATO-backed uprising in 2011 lead to the toppling of long-time dictator Muammar Gaddafi.
Hostilities in Es Sider and Ras Lanuf come only a day after reports from Turkish media that a Turkish-flagged tanker had been seized in the Libyan port of Zuwarah - a black-market trading hub - over disagreements over oil payments.
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