Libyas National Oil Corporation (NOC) issued a statement to its customers warning of entities offering Libyan oil in illegal deals after Islamist-led militias took control of key energy assets last week.
The notice reminds NOC clients that only NOC is legally authorised to sign off oil sales and that any deals done with other entities will not be valid.
It also states that any tankers that enter Libyan waters without consent will be considered rogue.
Libyas democratically elected parliament, the House of Representatives, is currently based in the eastern city of Tobruk, while a rival Islamist parliament, the General National Congress, has control of Tripoli.
The General National Congress is supported by Libya Dawn, an Islamist-led militia coalition, and took control of Tripoli and the surrounding area in August after Libya Dawn defeated a rival pro-government militia coalition in a battle for the capitals international airport.
Over recent weeks the power struggle between the two rival governments has increasingly focused on the countrys oil reserves.
On 10 November militants thought to be from Libya Dawn took control of Libyas largest oil field, El-Sharara, which is located in the countrys south.
The field has a capacity of 340,000 barrels a day (b/d).
Libya has seen huge disruption in its oil sector since the dictator Muammar Gaddafi was ousted in 2011 during the countrys civil war, with various armed groups taking control of ports, pipelines and oil fields in order to extract concessions from the government.
Recent disruptions including the shutdown at El-Sharara and a suspension of operations at the 120,000 b/d port of Hariga have reduced Libyan exports from around 900,000 b/d to around 500,000 b/d.
El-Sharara was shut for four months earlier this year due to protesters making financial and political demands.