Forces loyal to Libyan military strongman Khalifa Haftar say they have retaken key oil-rich areas in the country's east.
Ground, sea and air forces were engaged in the fight for sites at Ras Lanuf, Sidra and Ben Jawad from a rival Islamist militia, a spokesman said.
Meanwhile, Russia has denied reports that it has deployed special forces to the region in support of Gen Haftar.
Libya has been in chaos since the overthrow of Colonel Gaddafi in 2011.
The oil terminals had been seized by the Benghazi Defence Brigades (BDB) - a mix of militias that includes Islamists - earlier this month, which then handed them over to the Petroleum Facilities Guard, affiliated to the UN-backed unity government based in Tripoli.
Gen Haftar is allied to an administration based in the eastern city of Tobruk, which is challenging the authority of the UN-backed government.
Analysis - Amira Fathalla, BBC Monitoring
The clashes in Libya's vital Oil Crescent are likely to continue, locking the rival sides in a tug-of-war over power and resources. Neither side appears willing to give up its claim to the oil sites, Libya's economic lifeline.
Clashes are also likely further east in the country if the BDB militia follows through on its vow to advance on Benghazi. The city falls under the control of Gen Haftar's forces, which have fought to expel Islamist fighters there for over three years.
This would further polarise the rival political camps and could draw in more among the myriad of Libyan militias into a wider, protracted battle around the country's second city.
Russia's defence ministry denied allegations that it had special forces at an Egyptian base, some 60 miles (100 km) from the Libyan border.
"There are no Russian special forces in Sidi-Barrani. It's not the first time such leaks from anonymous sources to certain Western media have got people excited," defence spokesman Igor Konashenkov said.
The report by Reuters news agency, quoting unnamed Egyptian officials, had said that a 22-strong unit of Russian special forces were at the base.
The Egyptian military has also denied the reports.
Gen Haftar has held talks with senior Russian officials in recent months.
In January he was given a tour of a Russian aircraft carrier the Admiral Kusnetsov in the Mediterranean and spoke by video link to Russia's Defence Minister Sergei Shoigu.
In November last year he met Russian Foreign Minister Sergey Lavrov in Moscow and asked for Russian assistance in fighting Islamist militias in Libya, Reuters reported.
Libya remains regionally split with two centres of power that politically oppose each other, and a myriad of rival armed groups that the country's two governments cannot control.
Gen Haftar's forces, known as the Libyan National Army (LNA), have been battling Islamist and other militias in the area since forcing them out of much of the country's second city, Benghazi, in February 2016.
Extremist groups, including so-called Islamic State (IS), gained a foothold in Libya after Nato-backed forces ousted veteran leader Muammar Gaddafi in 2011.