TRIPOLI (Reuters) - Libya's Defence Ministry has authorised the military to use force to stop a North Korean-flagged tanker from loading crude oil at a rebel-held port, bypassing the Tripoli government, it said on Sunday.

The tanker docked on Saturday at the eastern terminal of Es Sider, one of three ports seized by rebels since August to press demands for autonomy and a bigger share of oil revenue. Local daily al-Wasat said the ship had loaded US$36 million (S$45.7 million) of crude.

The rebel oil sale illustrates the deepening turmoil in the OPEC producer, which has failed to rein in fighters who helped oust Muammar Gaddafi in 2011 but who now defy state authority.

In Tripoli, workers at a state oil firm that runs Es Sider port went on strike, urging the government to intervene because their colleagues were under duress from armed protesters.

"We are very angry at what is happening at Es Sider," said Mr Salah Madari, an oil worker in the capital. "The port's control officer is being held at gunpoint," he said, adding that gunmen had also forced a pilot to guide the tanker into dock.

Prime Minister Ali Zeidan said on Saturday the military would bomb the 37,000-tonne Morning Glory if it tried to leave the port, one of Libya's biggest oil terminals.

"The Defence Ministry issued orders to the chief of staff, air force and navy to deal with this tanker which entered Libyan waters without official permission," it said in a statement.

"The order authorises the use of force and puts the responsibility for any resulting damage on the ship owner."

Spokesmen for the state-run National Oil Corp (NOC) and for the protesters said the tanker was still docked at the port.

A Reuters reporter who visited Es Sider on Saturday evening saw a small number of protesters at the gate. One said their orders were not to let staff out until the loading was complete.

The rebels are led by former anti-Gaddafi commander Ibrahim Jathran, who once led a brigade paid by the state to protect oil facilities. He turned against the government and seized Es Sider and two other eastern ports with thousands of his men in August.

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. The region felt disadvantaged under Gaddafi and wants to restore the oil revenue share it received under his predecessor King Idris, who was toppled in 1970.

In January, the Libyan navy fired on a Maltese-flagged tanker that it said had tried to load oil from the protesters in Es Sider, successfully chasing it away.

It is very unusual for an oil tanker flagged in secretive North Korea to operate in the Mediterranean, shipping sources said. NOC says the tanker is owned by a Saudi company. It has changed ownership in the past few weeks and had previously been called Gulf Glory, according to a shipping source.

Libya's government has tried to end a wave of protests at oil ports and fields that have slashed oil output to 230,000 barrels per day (bpd), from 1.4 million bpd in July.

Western powers worry Libya will slide into deeper instability or even break apart.