Two employees have been killed in an attack on the Tripoli headquarters of Libyan crude exporter NOC.
The raid on Monday also injured 10 staff, who were taken to hospital.
NOC said armed attackers stormed the building at 9am local time.
There were a number of explosions inside, with a number of staff temporarily held hostage.
Security forces later arrived and cleared the building.
NOC chairman Mustafa Sanalla was inside at the time.
He said: “We solemnly mourn the death of those martyred by the terrorists and wish a speedy recovery to those injured.
"We praise our staff for their bravery in the face of unbelievable violence, and give thanks to the swift response of the emergency services and the security teams working to end this horrific situation."
He added: “Far more important than the considerable damage to our building is the human cost of such an attack. Our people are our most precious asset. NOC will not change in its mission because of such actions - we will stay strong and resolute."
Sanalla said there had been no impact on production and operations.
"This incident however serves to demonstrate the fragile security situation in our country and the need for additional measures to ensure NOC is able to withstand those that seek to halt Libya’s recovery," he added.
"We call on all parties to ensure that the perpetrators of such acts are brought to justice."
Some of NOC's terminals have been hit by fires and kidnappings in recent months as rival militias battle for control, limiting tanker loadings.
