* Govt forces would struggle against battle-hardened militias
* North Korean-flagged tanker seized by govt forces - minister
* Oil blockade has slashed vital govt revenues (Adds government forces seizing tanker, paragraphs 4-7)
By Ulf Laessing and Feras Bosalum
TRIPOLI, March 10 (Reuters) - Libya's parliament has ordered a special force to be sent within one week to "liberate" all rebel-held ports in the volatile east, officials said on Monday, raising the stakes over a blockage that has cut off vital oil revenue.
The conflict over oil wealth is increasing fears that Libya may slide deeper into chaos or even splinter as the fragile government fails to rein in dozens of militias that helped oust Muammar Gaddafi in 2011 but now defy state authority.
The rebels, who have seized three ports and partly control a fourth in the OPEC member country, said they had dispatched forces to central Libya to deal with any government attack.
With tension between the two sides escalating, government forces seized a tanker that had loaded crude worth $30 million at the rebel-held Es Sider port.
The North Korea-flagged tanker was undamaged and being escorted to western Libya, culture minister and government spokesman Habib al Amin told Reuters.
"The government announces that the tanker is now under control of the navy and the revolutionary forces, and on its way to a port controlled by the government," he said.
A spokesman for the rebels earlier denied they had lost control of the ship.
Even without any major military action, the escalation kills any hope of restoring oil exports soon. A wave of protests at oilfields and ports has reduced output to a trickle, undermining state authority as oil is the main revenue source supporting the budget and basic food imports.
The head of parliament, who has quasi-presidential powers, ordered the formation of a force made up of regular soldiers and allied militias to take back the ports, which previously handled a total of more than 700,000 barrels of oil per day.
The operation will start within one week, parliament head Nuri Ali Abu Sahmain said in a decree published by spokesman Omar Hmeidan. "The force will be set up to liberate the ports and end the blockage," Hmeidan told Reuters.
Prime Minister Ali Zeidan, who said on Saturday the tanker, Morning Glory, would be bombed if it tried to export oil, is now in a much stronger position with the parliament throwing its weight behind military action.
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.
Still, the force will be drawn from cities such as Misrata that are home to fighters who saw battle in the civil war, according to the decree. Misrata forces were sent earlier this year to assist with clashes deep in Libya's south.
Abb-Rabbo al-Barassi, self-declared prime minister in the rebel movement, called on "all honourable men" in the east to join his forces, a rebel television station quoted him as saying.
The rebels, made up of former oil security guards, said they had sent forces by land and sea to central Libya to confront any government attackers. The east and west are connected by a coastal highway road.
"We have sent land forces to defend Cyrenaica to the west of Sirte ... and we also have boats patrolling regional waters," Essam al-Jahani, a member of the rebels' leadership team, told Reuters.
Cyrenaica is the historic name for eastern Libya, for which the rebels demand political autonomy and a share of oil revenues like under King Idris, who preceded Gaddafi.
Sirte, Gaddafi's birthplace, is a coastal city that forms a stronghold for Islamists and militias in central Libya.
FULL CONFRONTATION UNLIKELY
While the navy opened fire on a Maltese-flagged tanker trying to approach Es Sider in January, analysts say a full military confrontation with the rebels would be unlikely.
Any bloodshed might boost calls for a federal state like under Idris, sharing power among the three historic regions Cyrenaica, Tripolitania in the west and the southern Fezzan - or even for secession.
The protesters are led by a former anti-Gaddafi commander, Ibrahim Jathran, who until summer was in charge of protecting oilfields and ports until he turned against the government.
Jathran's campaign seeking more rights for Libya's underdeveloped east has won him some sympathy, but 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, according to a statement read out on television.
The United Nations' special envoy to Libya, Tarek Mitri, told the U.N. Security Council on Monday that the loading of oil onto the North Korean-flagged vessel "constitutes an illegal act and violates Libya's sovereignty over its ports and natural resources".
On Sunday, Tripoli said the navy and pro-government militias had sent boats to stop the 37,000-tonne tanker from leaving. The vessel arrived at Es Sider on Saturday.
It is unusual for a tanker flagged to secretive North Korea to be in the Mediterranean. Shipping sources said it was a flag of convenience to keep the ship's ownership secret.
State-run National Oil Corp, which says the tanker is owned by a Saudi firm, said it would sue anyone trying to buy the oil.
In a rare bright spot, NOC managed to restart the southern El Sharara oilfield after a protest ended there, a spokesman said. It is now pumping 150,000 barrels per day (bpd) but might reach full capacity at 340,000 bpd by Tuesday afternoon. (Reporting by Ulf Laessing, Feras Bosalum and Ayman al-Warfalli; additional reporting by Michelle Nichols at the United Nations; Editing by Dale Hudson)