0505 GMT May 24, 2019
The seizure of Sharara oilfield prompted the company to declare a "force majeure" on exports from the field.
"National Oil Corporation (NOC) hereby declares a state of force majeure at the Akakus-operated Sharara oil field as of Sunday," the company said in an online statement.
Akakus is a joint venture between NOC, Spain's Repsol energy company, France's Total, Austria's OMV and Norway's Statoil.
It produces some 315,000 barrels per day, out of Libya's current output of one million bpd.
Force majeure, invoked in exceptional circumstances, exempts the company from liability in case of non-compliance with oil delivery contracts.
Oil facilities in Libya are regularly targeted by rival armed groups, local militias or tribes vying for power or pushing for their social demands to be met.
NOC, which is reviewing procedures to evacuate employees, urged local authorities to "act in the national interest and return security to the site."
It also called on the group to leave the oilfield "immediately without pre-condition," ruling out negotiations with a militia.
"The presence of this group is a real threat to the field and to the future of our country," said NOC chairman Moustafa Sanalla. "I want to be clear, this militia has to leave the field immediately."
Libya has plunged into chaos following a 2011 NATO-backed uprising that toppled and killed former dictator Muammar Gaddafi.
Two competing administrations, rival militias, tribes and militants have been vying for control of territory and the country's vast oil wealth.