An unnamed armed group shut a valve on a pipeline from a major Libyan oil field, causing a drop in production, the National Oil Corporation (NOC) has said.
The disruption threatens production from the Al-Sharara field, a key source of crude for Libya, an OPEC member that holds Africa's largest proven oil reserves.
Armed groups and protesters frequently target oil fields, pipelines and export terminals to press political or economic demands.
The NOC said in a statement that the closure affected a pipeline linking the Al-Sharara field, located 900 kilometers (560 miles) south of Tripoli, to the coastal city of Zawiya, home to one of the country's main export terminals. It did not name the group responsible.
If the shutdown continues, the company could be "compelled to declare force majeure", a legal provision that allows suppliers to suspend contractual obligations because of circumstances beyond their control.
The company also warned that the Zawiya refinery, about 45 kilometers west of the capital Tripoli, could be forced to shut down the disruption continued.
Despite its vast energy wealth, Libya has been beset by violence and instability since the overthrow of Muammar Gaddafi in 2011.
It remains divided between the UN-recognised government in the capital Tripoli led by Prime Minister Abdulhamid Dbeibah, and the rival administration in the east backed by military strongman Khalifa Haftar.
