* Tanker's escape from navy would be huge blow to govt
* Cargo is first oil sale from rebels demanding autonomy
* Clashes between rebels, pro-govt militias reported near port
* Western powers worry Libya will slide into instability
By Ayman al-Warfalli
ES SIDER, Libya, March 11 (Reuters) - A North Korean-flagged tanker that loaded crude oil at a rebel-held port in eastern Libya is in international waters after escaping the navy following a brief firefight, rebels at the harbour and a state oil company official said on Tuesday.
In another worrying sign for the Tripoli government, gunmen blocked the gate to an airbase in Sirte, a city located close to the Es Sider port where rebels have sought to sell oil independently to get a greater share of Libya's mineral wealth.
If the tanker's escape is confirmed, it would be a huge embarrassment for Tripoli.
Top officials including Prime Minister Ali Zeidan said late on Monday the navy had seized the ship after the vessel loaded oil in Es Sider and was escorting it to a harbour controlled by the government.
The conflict over oil wealth is increasing fears that the OPEC member country may slide deeper into chaos or splinter as the government fails to rein in dozens of militias that helped oust Muammar Gaddafi in 2011 but now defy state authority.
A Reuters reporter at Es Sider port said there was no sign of any tanker but was unable to get more information on the whereabouts of the ship.
Mohammad Hitab, spokesman for the state-run al-Waha Oil Company operating the port, said: "The tanker left and is now in international waters."
He said he did not know the destination of the tanker, which Libyan officials say is owned by a Saudi company.
Several rebels and port workers sympathising with them said the ship had left early in the morning, with boats escorting it into international waters.
"We escorted the tanker out of the port at 3 a.m.," said a rebel fighter at the harbour. "We came under fire from light arms (from the navy) but there were no casualties."
There were around 10 rebel vehicles mounted with guns inside the port, one of Libya's biggest export terminals.
The Libyan navy and oil-related government officials declined to comment. An official in Zeidan's office said the government would issue a statement later.
Local residents said gunmen from the rebel force had entered the central city of Sirte and parked their combat vehicles at the entrance to the civilian airport and an airbase. The air force base is the only major one in the area.
A rebel spokesman said his forces and pro-government militias had clashed in Sirte but Reuters could not verify this.
On Monday, Libyan officials said the government would assemble forces to "liberate" all occupied ports, raising the stakes over a blockage that has cut off vital oil revenue.
In Tripoli and the western city of Misrata, armed young men could be seen gathering to join a government force, residents said.
The rebels also demand autonomy for the east, which had been neglected under Gaddafi as he concentrated power and wealth in Tripoli as well as his home region of Sirte.
The rebels, who have seized three ports and partly control a fourth in the North African country, said they had dispatched forces to central Libya to deal with any government attack.
Libya has been trying to rebuild its army since Gaddafi's ousting, but analysts say it is not yet a match for battle-hardened militias that fought in the eight-month uprising that toppled the dictator.
Ibrahim Jathran, the leader of the protesters, is a former anti-Gaddafi commander who was in charge of protecting oilfields and ports until he turned against the government in the summer.
His campaign to seek 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. (Reporting by Ayman al-Warfalli in Es Sider, and Feras Bosalum; and Ulf Laessing in Tripoli; Editing by Dale Hudson)