Offensive launched by UN-backed Tripoli government against rival administration’s fighters who seized ports last week.
Eastern Libyan forces said they had re-established control over two oil ports where an ousted faction launched a counterattack on Sunday, briefly seizing one of the terminals.
The Petroleum Facilities Guard - an armed group led by Ibrahim Jathran and loyal to the Tripoli-based government - said it attacked two of the oil ports captured by forces of rival military leader Khalifa Haftar.
Last week, Haftar’s forces seized the ports of Ras Lanuf, Al-Sidra, Zuwaytina, and Brega in the so-called oil crescent along the coast, which were then handed to the National Oil Corporation.
Muftah al-Muqarief, who heads oil guards loyal to Haftar, said the assault on the ports was launched from the west by "militias backed by outlaws".
"We repelled the attack and we are chasing them in the region," he said, adding "some" assailants had been captured.
The Tripoli-based Government of National Accord (GNA) has struggled to assert its authority over the country since it was formed in December last year.
Oil is Libya’s key asset, and revenue from crude exports is vital if the GNA is to rebuild the economy and infrastructure of the North African nation, ravaged by violence and conflict between factions since the 2011 uprising that led to the toppling of the late leader Muammar Gaddafi.
The loss of the ports was a blow to the GNA, which set up base in the capital in March, months after it was created as the result of a UN-backed power-sharing agreement.
The fighting came as the state-run National Oil Corporation (NOC) prepared to restart oil exports from the ports, blockaded for several years.
"This situation highlights the contradiction and complication of the NOC," reported Al Jazeera’s Mahmoud Abdel Wahed from Tripoli.
"NOC is supposed to be loyal to the Government of National Accord, but it recently welcomed Haftar’s control of the oil terminals."
Haftar - who sees himself as Libya’s saviour after driving out armed groups from most of the country’s second city Benghazi - is the most powerful backer of the rival administration in the east.
Days after the ports fell under his control, the east-based parliament promoted Haftar to field marshal from his previous rank of general.
The GNA is the centrepiece of UN efforts to restore stability and forge a central authority capable of tackling the twin scourges of a significant Islamic State group presence and rampant people trafficking across the Mediterranean to Europe.