Libya's state oil company said on Tuesday that the 120,000 barrels per day Zawiya oil refinery complex in a western city of its capital Tripoli was severely damaged due to some skirmishes late Monday.
The NOC said "Those involved held no regard for the lives of workers at the site."
Eight oil storage tanks and five other tanks for oil and chemical products were damaged and were leaking while the electrical transformer of the power supply system was also knocked out.
According to the NOC chief, Mustafa Sanalla "The infrastructure of the oil sector represents the lifeblood of the Libyan state therefore"…vandalising these facilities, destroying them, or exposing their workers to danger, is a crime that cannot be tolerated."
Images posted on social media showed gunmen exchanging heavy fire around the refinery, but neither the identity of the groups nor the cause of the clashes is known.
Libya's oil sector, the country's only vital income, has been disrupted by unrest since the falling of long-time dictator Muammar Gaddafi a decade ago.
Zawiya city, some 40 km (25 miles) in west of Tripoli, is the home of Libya's biggest functioning refinery. It is connected to Sharara field, which has a capacity of around 300,000 bpd.
Reports of security unrest in the city were circulated on the internet, but no immediate comments were available by authorities.