A North Korean-flagged tanker has finished loading crude at the Libyan rebel-held port of Es Sider but has not left the terminal, oil officials said on Monday, after the government threatened to bomb the ship.

Tripoli has said it will attack the Morning Glory should the tanker try sailing off with crude from the eastern port, one of three seized by armed protesters demanding a greater share of oil revenues and political autonomy.

"The tanker has finished loading but is still anchored in Es Sider," a senior oil official said. The value of the cargo is close to $30 million.

State-run National Oil Corp (NOC) said it would sue anyone trying to buy the oil. "NOC will ... enforce its rights over the cargo and hold responsible all the parties participating in illicit transactions relating to it in any jurisdiction, both within and outside of Libya," it said in a statement.

The 37,000-tonne vessel docked at Es Sider on Saturday. NOC says the ship is owned by a Saudi firm, but it is unclear where it might be trying to sail.

On Sunday, the government said the navy and pro-government militias had dispatched boats to stop the tanker from getting out. The rebels said any attack on the vessel would be "a declaration of war".

The conflict over oil wealth is increasing fears of Libya sliding deep into chaos or even splintering as the fragile government fails to rein in fighters who helped oust Muammar Gaddafi in 2011 and who now defy state authority.

The protesters are led by a former anti-Gaddafi commander, Ibrahim Jathran, who seized with thousands of his men three eastern ports in the OPEC member country.

While the navy opened fire on a Malta-flagged tanker trying to approach Es Sider in January, analysts say a full military confrontation with Jathran would be unlikely.

Prime Minister Ali Zeidan has been weakened by political infighting with parliament and reluctant to use the military to clear the seized ports, although talks to end the blockage have gone nowhere.

Libya has been trying to rebuild its army since Gaddafi's overthrow, but analysts say it is not yet a match for battle-hardened militias that fought in the eight-month uprising that toppled him.

While Jathran's campaign seeking more rights for Libya's underdeveloped east has won him sympathy, many people dismiss him as a tribal warlord with no political vision. Libya's top Islamic clerics urged militias who had helped topple Gaddafi to assist the government in trying to stop the tanker.

Tripoli has held indirect talks with Jathran, but fears his demand for a greater share of oil revenue for eastern Libya might lead to secession. – Reuters