“Eight Libyans, three Filipinos and two Ghanians were killed in the attack” at the Al-Mabruk field, said officer Hakim Maazzab, the head security guard at a nearby oil complex.
“All of them had their throats slit apart from one Libyan, who was shot dead.”
Previously, the state-owned National Oil Company had said staff were evacuated from the field.
“An unidentified armed group attacked the field but personnel were evacuated in time,” NOC spokesman Mohamed al-Harari told AFP.
A spokesman for the guards at Libya’s oil installations, Ali al-Hassi, accused militants loyal to the Islamic State group of carrying out the attack, without providing details.
Al-Mabruk sits some 100 kilometres (60 miles) south of the coastal city of Sirte, the hometown of slain dictator Moamer Kadhafi.
French government spokesman Stephane Le Foll earlier said Paris was seeking to confirm casualties in the attack but added that “no Westerners” had been killed.
“We have been informed that an armed group has attacked the Mabruk site,” a spokeswoman for Total said, adding that the company had previously withdrawn all of its personnel.
“Total is not the operator. The site is operated by the Mabruk Oil Operation, which is managed by the NOC,” she added.
Since Kadhafi’s overthrow in a NATO-backed revolt in 2011, Sirte has become a stronghold of extremist groups, including Ansar al-Sharia, which is blacklisted by the United Nations and the United States for its links to Al-Qaeda.
There was no production from the Al-Mabruk field at the time of the attack because of restricted export capacity at terminals on the coast.
An Islamist-backed militia alliance, which controls the capital Tripoli and third city Misrata, launched an offensive in December to try to capture the export terminals from forces loyal to the internationally recognised government, causing significant damage.
Oil is Libya’s main natural resource, with a pre-revolt output of about 1.6 million barrels per day (bpd), accounting for more than 95 percent of exports and 75 percent of budget revenues.
But production fell to around 350,000 bpd in December when the Fajr Libya (Libya Dawn) alliance attacked the terminals.
The jihadist Islamic State group that has seized chunks of Iraq and Syria is thought to have gained a foothold in Libya and claimed last month’s attack on a luxury hotel in Tripoli that killed nine people, including five foreigners. (AFP)