Malta has barred an Indian-flagged tanker from its territorial waters because it is carrying an allegedly illegal shipment of oil from Libya.
The Distya Ameya was anchored 12 kilometers off Malta's southeastern coast Tuesday.
It is loaded with 650,000 barrels of crude oil sold by a Libyan administration in Tobruk. The Bloomberg news agency says the buyer is a company based in the United Arab Emirates.
A rival Libyan administration based in Tripoli calls the oil sale illegal, saying its National Oil Corporation is the only internationally recognized body that can sell Libyan oil.
But the rival eastern corporation says it is obligated to fulfill all contracts.
Neither Libyan administration has accepted the authority of the U.N-backed unity government for Libya.
State Department spokesman John Kirby said Tuesday the U.S. is concerned about the purchases of Libyan oil outside of legitimate channels. He said all oil sales must come from the Tripoli-based oil company "to maintain the credibility and stability of Libya's oil in international oil markets."
Libya has been in political limbo since longtime dictator Moammar Gadhafi was toppled and killed in 2011.
Rival tribes and former opposition leaders have jockeyed for control of the Libya oil industry, on which the country's economy depends.