Two guards and a soldier were killed and another four people kidnapped in a suspected Islamic State attack targeting Libya’s Zella oilfield, a security source said.
The death toll was confirmed by the National Oil Company (NOC) which condemned the attack in a statement.
The attackers struck at an entrance gate to the field, near Zella about 760 km south-west of Tripoli, before fleeing, according to the source and local residents who asked not to be named.
Islamic State claimed responsibility through its Aamaq news agency.
The Zella field belongs to Zueitina Oil Company, which pumped 19,000 barrels per day on average in the last quarter of 2018 across all its fields.
An engineer told Reuters workers at the field were safe and facilities were not damaged.
Libya’s NOC chief said continued instability in the country could cause it to lose 95% of oil production.
Speaking in Saudi Arabia ahead of a ministerial panel gathering of top OPEC and non-OPEC producers, Mustafa Sanalla confirmed the Zella attack.
Islamic State has been active in Libya in the turmoil since the overthrow of Muammar Gaddafi in 2011. The militant group took control of Sirte in 2015 but lost it late in 2016 to local forces backed by US air strikes.
In the last two years, the group targeted three state institutions in Tripoli, home of the UN-backed government of national accord led by Prime Minister Fayez Serraj.
Saturday’s assault took place as General Khalifa Haftar’s Libyan National Army (LNA), allied to a rival administration in eastern Libya, mounts an offensive to control Tripoli.