Oil output has fallen to a fifth of what Libya used to produce as major oil ports and fields have been knocked out in a conflict Western powers fear will tear apart the country.
"Unknown gunmen stormed the Mabrook oilfield last night," National Oil Corp (NOC) spokesman Mohamed El Harari said, without providing details.
Operations at the field, located south of Sirte, were closed following clashes which shut the Es Sider oil port in December. It used to pump 40,000 barrels a day. France`s Total, which operates the field with NOC, said it had already withdrawn staff from the site.
It was not clear whether NOC had employed expatriate staff at the field located in a remote corner of the desert nation.
Ali al-Hassi, spokesman of an oil guard force allied to the internationally-recognized government based in the east since a rival group seized Tripoli in August, blamed Islamists for the attack.
"The field is outside of our control," he said.
It was not immediately possible to verify his view that Islamists were involved. Militants claiming links to Islamic State, a group which controls parts of Syria and Iraq, last week claimed an attack on a luxury hotel in Tripoli killing at least nine people.
Officials of a rival government in Tripoli denied the claim, blaming "Gaddafi loyalists" for the storming of the hotel.
Western powers and Libya`s neighbors have been worried about a spread of radical Islamists in the desert nation.
Sirte is home to members of the Ansar al-Sharia Islamist group. The region north of it has seen fighting as armed groups vie for control of the Es Sider and Ras Lanuf oil ports.
OPEC member Libya`s oil output has fallen to around 350,000 barrels per day, the latest data shows, from 1.6 million pumped during Gaddafi`s reign.