National Oil Corporation Chairman Mustafa Sanallah told the Libya Al-Ahrar television channel late Sunday that the Sharara oil field was occupied by the Russian security company Wagner's mercenaries 25 days ago.
Noting that the damage caused to the petroleum industry since 2012 was $231 billion, Sanallah said that Wagner militias should leave the region immediately.
The Ras Lanuf and Sidra oilfields are estimated to account for more than half of Libya's oil production.
Pro-Haftar forces blockaded the main oil terminals in eastern Libya the day before a summit in Berlin on Jan. 19 that called for the end of foreign interference in the conflict and a resumption of the peace process.
Exports were suspended at the ports of Brega and Ras Lanuf.
Meanwhile, Russia continues to send mercenaries to Libya from Syria, Anadolu Agency reported citing local sources.
Some 300 Syrian Shabbiha militants from Deir el-Zour province, including those from Iranian-backed Fatimiyyun, Zaynabiyyun and al-Quds brigades, were sent to Libya.
The militias are expected to receive $1000 to $1500 per month. They were taken to Russian military bases in Latakia to receive training, sources said.
There are eight Daesh terrorists among the mercenaries sent to Libya, sources added.
Russia had also been trying to recruit fighters from various Arab tribes in the terrorist PKK/YPG-held Hasakah province to fight in Libya.
This is not the first time Russia has sent mercenaries to Libya from Syria and other places. A similar transfer of some 600 mercenaries from Syria's Homs province took place earlier.
Russian mercenaries called the "Wagner Group" also support Haftar's forces in Libya.
The United Nations have documented the involvement of foreign mercenaries in the Libyan conflict.
However, some fighters refuse to accept the offer after realizing that they would be fighting against the Libyan government.
Libya has been torn by a civil war since the ouster of Moammar Gadhafi in 2011. The country's new government was founded in 2015 under a U.N.-led agreement, but efforts for a long-term political settlement failed due to a military offensive by Khalifa Haftar's forces.
The U.N. recognizes the Libyan government headed by Fayez Sarraj as the country's legitimate authority in Tripoli.
The government launched Operation Peace Storm against Haftar in March to counter his attacks on the capital since April 2019, and recently liberated strategic locations including al-Watiya air base and the city of Tarhuna, Haftar's final stronghold in western Libya.