Production has stopped at Libya’s major El Sharara oilfield after an armed group seized control of the field late Tuesday.
There were reports of gunfire and looting at the field but no official details have been released.
Thirteen people were injured during the attack.
It is unclear who the armed group is affiliated with.
Workers were trapped inside buildings at the site for hours after the initial attack, Reuters said.
Spain’s Repsol and Libya’s state owned National Oil Corporation operate the field.
The Libyan oil ministry said El Sharara produces at least 200,000 barrels per day.
El Sharara has a total capacity of 340,000 barrels per day but has been running below capacity after some well heads were lost when the field was closed down by protesters earlier this year.
The field is the largest producing field in Libya and is located in the southern part of the country in the Murzuq Desert.
Rival tribes have been clashing in the region since Muammar Gaddafi was deposed three years ago.
Protesters closed down the field twice during the last year to pressure the government to give groups from the region greater political recognition and rights.
The current shut down is also expected to hit output at the 120,000 bpd Zawiya refinery that is fed by El Sharara.
Libya is currently OPEC’s smallest producer but has the seventh largest proven reserves of OPEC’s twelve member states.