The Benghazi Defence Brigades attacked Ben Jawad town near the coastal "oil crescent", where Haftar had seized four export terminals from pro-government forces in September.
The alliance of Islamist and tribal fighters was then repelled by Haftar's forces, Colonel Moftah el-Magarief, head of an oil facilities guard under Haftar's control, told AFP.
"We have taken control of Ben Jawad and seized equipment and prisoners from the Benghazi Defence Brigade," he said.
"The air force targeted equipment belonging to the attacking force and we can confirm that all the oil fields and terminals are under our forces' control."
An engineer at the Al-Sidra port, 30 kilometres (20 miles) east of Ben Jawad, said an aircraft belonging to Haftar's forces had bombed a column of military vehicles belonging to the Benghazi fighters.
"The Benghazi Brigades (then) targeted us with Grad rockets," he said.
Libya's oil exporting region is bitterly contested between the country's internationally recognised Government of National Accord and a rival administration in the east, supported by Haftar.
Pro-GNA forces this week ousted the Islamic State group from its coastal bastion of Sirte, between Tripoli and the oil crescent, after a seven-month battle.
Experts have raised fears that having won in Sirte, GNA forces would move to retake the oil crescent, triggering renewed fighting between forces allied with the two rival governments.
But the GNA on Wednesday denied reports it had ordered any group to advance on the area.
In a statement, it said it was "in no way involved with the military escalation on Wednesday in the oil crescent".
Rocked by chaos and divisions since the fall of dictator Moamer Kadhafi in 2011, Libya desperately needs to relaunch its oil exports, the backbone of its economy.
The head of its National Oil Corporation warned in September that the country faced financial collapse unless it swiftly resumed exports.