The Libyan National Army on Sunday announced a "major offensive" to drive rival groups from the country's north-eastern oil crescent, where militias have captured two vital export terminals.
Armed groups including the Petroleum Facilities Guard led by Ibrahim Jadhran seized the Ras Lanuf and Es Sider terminals from the LNA on Thursday. One oil storage tank caught fire in the fighting and another was reported to be ablaze on Sunday.
"We have launched a major offensive supported by the army and air force to drive out the militias of Jadhran and his allies," LNA spokesman Ahmed Al Mesmari said.
The Petroleum Facilities Guard controlled the terminals, about 650 kilometres east of Tripoli, for years following the 2011 killing of longtime Libyan dictator Muammar Qaddafi, but were eventually forced out by the LNA.
The LNA, led by Field Marshal Khalifa Haftar, controls most of eastern Libya and is opposed to an internationally recognised government based in Tripoli, which has itself condemned Thursday's militia attacks.
On Thursday, Jadhran said in a video that he had formed an alliance to retake oil terminals seized by the LNA in September 2016.
The LNA's air force on Sunday told residents in the oil crescent to stay away from "areas where the enemy gathers, munition stores and sites with military vehicles".
"Fighter [planes] are carrying out raids against terrorist positions and gatherings in the operational military zone stretching from Ras Lanuf to the edge of the city of Sirte," the air force said on its Facebook page.
The Red Crescent in Ajdabiya, 150km east of Ras Lanuf, said on Friday that it had received 28 bodies, without specifying to which group they belonged.
Libya's oil authority said on Saturday that a storage tank had been "significantly damaged" in the attack on Ras Lanuf and Es Sider.
The National Oil Corporation (NOC) called for the "immediate and unconditional surrender" of Jadhran's militia to "prevent an environmental disaster and further destruction of key infrastructure".
The NOC said it had halted exports from Ras Lanuf and Es Sider after the attack on Thursday and warned that output could fall by up to 400,000 barrels per day if the shutdown continued, leading to a "national disaster".
Libya's economy relies heavily on oil. Daily production of 1.6 million barrels under Qaddafi fell to about 20 per cent of that level after the 2011 uprising. It had recovered to more than one million barrels per day by the end of last year.