* Navy, pro-government militias send boats to Es Sider
* Armed protesters try to sell oil outside state control
* Three Libyan ports in rebel hands since August (Adds minister, navy sends boats, background)
By Ulf Laessing and Feras Bosalum
TRIPOLI, March 9 (Reuters) - Libya's navy and pro-government militias have dispatched boats to a port held by armed protesters to stop a North Korean-flagged tanker from leaving with crude sold without government permission, officials 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 $36 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 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.
There was no sign of any immediate military action. But the navy and allied militias have sent several boats to stop the tanker from leaving, said Culture Minister Habib al-Amin.
He and other government officials did not elaborate at a news conference, but Libyan news websites showed some small boats close to a tanker which they said was the Morning Glory.
"Several navy boats have been dispatched. Now the tanker's movements are under complete control and nobody can move it," said Amin, who acts as informal government spokesman. "The tanker will stay where it is."
"All efforts are being undertaken to stop and seize the tanker, if necessary by a (military) strike, if it does not follow orders," he said, adding that state prosecutors would treat the loading of the crude as smuggling.
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.
There was no immediate reaction from the protest movement made up of thousands of former state oil guards led by Ibrahim Jathran, who fought Gaddafi troops in 2011 in the east.
FEARS OF SECESSION
The Defence Ministry earlier issued orders to the chief of staff, air force and navy to deal with the tanker. "The order authorises the use of force and puts the responsibility for any resulting damage on the ship owner," it said in a statement.
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.
Jathran 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. (Additional reporting by Ayman al-Warfalli; Editing by Alistair Lyon)