Libya's 120,000 barrels a day Zawiya refinery has been shut down after storage was damaged in fighting between armed groups, an oil ministry official said on Thursday.
The El Sharara field feeding the refinery also remains closed, said Ibrahim al-Awami, head of the inspection and measurement department at the oil ministry.
He said he did not know when the refinery and oil field would resume work.
The closures are a big blow for the government because the refinery supplies the capital Tripoli and the rest of western Libya with fuel products.
The field closure will bring Libya's oil production down to around 670,000 barrels a day, based on production of 870,000 bpd reported by state-run National Oil Corp (NOC) on Sunday. No output update has been given since then.
Fighting hit the western town of Zawiya in the past few days with at least a rocket hitting a refinery storage tank connecting to the southwestern El Sharara field.
An armed group from Misrata which took the capital in August has been trying in the past few weeks to take an area west of Tripoli held by the rival Warshefana group.
Libya's oil industry had revived in the past two months after major oil ports in the east reopened following the end of a blockage by a rebel group demanding regional autonomy.
Western powers fear Libya might become a failed state as the weak government is unable to control militias which helped to topple Muammar Gaddafi in 2011 but have now turned their guns on each other.