Libya’s prime minister has threatened to bomb a North Korean-flagged tanker if it tries to export oil from a rebel-controlled port.
Ali Zeidan said that orders had been issued to arrest the tanker’s crew.
Earlier separatist militants said they had started loading oil on to the tanker at Sidra port, east of Tripoli.
The rebels have seized three major ports since August, to demand more autonomy, hampering the government’s ability to revive oil exports.
The Morning Glory docked at Sidra port earlier on Saturday, after a failed attempt to dock on Tuesday.
A local TV station showed footage of the rebels holding a ceremony to celebrate.
“We started exporting oil. This is our first shipment,” a rebel spokesman said.
Libyan officials said the rebel move was an “act of piracy”.
Analysts have said it is unlikely the ship is owned or controlled by Pyongyang.
Lawrence Dermody, a researcher in illicit trafficking for the Stockholm International Peace Research Institute, told the BBC: “It’s much more likely that it’s a flag of convenience,” adding that “it’s really not a common flag – even in the Middle East.”