Libyan Prime Minister Ali Zeidan threatened to use force against a North Korean-flagged ship, Morning Glory, that's been docked at the rebel-held port since last week. The U.S. government said it has a stake in Libyan crude oil though a partnership with the Libyan National Oil Corp.
"Any oil sales without authorization ... places purchasers at risk of exposure to civil liability, penalties and other possible sanctions in multiple jurisdictions," U.S. State Department spokeswoman Jen Psaki said in a statement Sunday
Libyan militia leader Ibrahim Jadran seized the port in eastern Libya in July. He told CNN in January the move was taken to counter corruption in Tripoli.
Ongoing security issues and internal rivalries have hurt Libya's oil production. Last week, British energy company BP said was exploring other options for exploration drilling onshore in Libya because of security concerns.
The Organization of Petroleum Exporting Countries said Libyan oil production more than doubled from Dec. 13 to Jan. 14 to 510,000 barrels per day, though that's well below the pre-civil war peak of 1.6 million bpd.