The Morning Glory docked at Sidra port earlier on Saturday, after a failed attempt to dock on Tuesday.
“We started exporting oil. This is our first shipment,” a rebel spokesman said.
The rebels demand more autonomy – and oil wealth – for Libya’s east.
Analysts have said it is unlikely the ship is owned or controlled by Pyongyang.
Libya’s state-owned National Oil Corp (NOC) had warned tankers against approaching the port, and two others in Libya’s volatile east that are also controlled by armed groups.
It is not the first attempt to ship oil from the rebel-controlled port.
On Monday, the Libyan navy ship Ibn Auf fired warning shots at a Maltese-flagged oil tanker to prevent it from docking and loading oil.
The owners of the ship complained it was fired on in international waters.
Libya’s government has tried to end a wave of protests at oil fields and ports, which have slashed vital oil revenues, but there has been little progress in indirect talks between the government and former militia leader Ibrahim Jathran, now leading the protests.
His men seized three eastern ports last year, which previously accounted for 600,000 barrels of oil a day.
Libya is struggling with armed groups and tribesmen who helped topple Muammar Gaddafi in 2011 but who have kept their weapons.
Ibrahim Jathran’s demands include an independent commission representing the three regions of Libya. He wants the commission to supervise the sale of oil and ensure the east gets a fair share of the revenue.
The government has so far not acted on threats to retake Sidra, or other rebel-controlled ports.
Libya’s oil output has slowed to a trickle since the protests started in July last year, depriving the OPEC producer of its main budget source.
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