A North Korean-flagged tanker that loaded crude oil from a rebel-held terminal in eastern Libya has escaped navy warships deployed to intercept it and is now in international waters, rebels and a state oil company official said.

The Morning Glory , which docked in al-Sidra port on Saturday and is reported to have taken on at least 234,000 barrels of crude, is the first vessel to have loaded oil from a rebel-held port since the revolt against the Tripoli authorities erupted last July.

"The tanker left and is now in international waters," Reuters quoted Mohammad Hitab, spokesman for the state-run al-Waha Oil Company operating the al-Sidra port, as saying.

Rebels demanding a bigger share of oil revenue and political autonomy in the east seized three ports last summer and partly control a fourth.

The central government had threatened armed action , even an air strike, to prevent the tanker from getting away with its cargo of oil bought from the rebels' self-declared autonomous regional government without the authorisation of the state-owned Libyan National Oil Corporation.

Members of the General National Congress (GNC), Libya's highest political authority, said that bad weather prevented the navy's small vessels from following the huge ship out into the Mediterranean.

Abdelkader Houili, who sits on the GNC's energy committee, told Al-Nabaa television that the navy's warships, which mainly consist of fast patrol boats, had been forced to sail close to the coast because of the weather.

"The tanker then took advantage of the gap to head for the open sea," he said.

The tanker's escape is a new humiliation for the Tripoli authorities who have been battling to assert control over much of the country since the NATO-backed 2011 revolt that ended the 42-year rule of Muammar Gaddafi.

In eastern Libya in particular, a myriad of former rebel fighters have carved out their own fiefdoms.