Libya's recognized government warned its security forces would seize any tankers approaching the Ras Lanuf terminal without permission, saying any attempt to make oil deals with the rival government in Tripoli would be "piracy".
The warning over Ras Lanuf illustrates how the OPEC country's oil industry is caught up in a power struggle between the two rival governments and their armed forces, who have each appointed competing figures in the state oil company.
The internationally recognised government and elected parliament has operated in the east since last year, when an armed faction called Libya Dawn took over the capital, set up its own government and took control of ministries in Tripoli.
Earlier this week, the National Oil Corporation (NOC) based in Tripoli said it was lifting the force majeure measure on the Ras Lanuf oil terminal which had been shut since December because of fighting between the rival factions.
But Ras Lanuf is protected by an armed force allied to the internationally recognised government, which has appointed its own NOC chief who claims to represent the country's industry.
The recognised government's NOC chairman, Yousef Bu Saifi, told Reuters the force majeure was still in effect and orders had been given to the Petroleum Facilities Guard or PFG at Ras Lanuf.
"We have asked the PFG to intercept any tankers trying to approach the oilfields for security and technical reasons. The force majeure is still in place," he said.
Hatem Oraibi, spokesman of Prime Minister Abdullah al-Thinni's recognized government, said any tankers dealing with the Tripoli government could be considered as committing piracy.
Oil guards at Ras Lanuf say they have received orders to warn vessels away and to arrest crew if they did not comply.
Foreign vessels off Libya's coast have in the past come under attack from Libya's armed factions, often because they are believed to be carrying arms or supplies for the rival group.
Four years after the civil war ousted Muammar Gaddafi, Libya is in chaos after brigades of former rebels who once fought together have steadily turned against one another.
Libya's oil industry has been badly hit. Strikes, protests and the conflict have closed down several major oilfields as well as ports. Production is less than half the 1.6 million barrels per day the North African state produced before 2011.