(Adds workers union, details, background)
By Feras Bosalum and Ulf Laessing
TRIPOLI, Dec 10 (Reuters) - Leaders of a movement seeking autonomy for Libya's eastern Cyrenaica region said on Tuesday they could allow oil exports to resume on Sunday from several ports, if Tripoli meets their demands and allows the region to take its share of crude.
But highlighting chaos in the OPEC producer, Libya's oil workers' union said it would stop any resumption of exports unless their pay demands were met by the central government.
Western powers worry Libya will slide into greater instability as the government struggles to rein in militias and tribes who helped topple Muammar Gaddafi but kept their guns.
Public pressure has been building on the movement to reopen oil ports it seized as exports have dried up, removing a key element in Libya's budget.
But it was unclear whether three months of standoff with the central government would end as the movement, which demands more power and oil revenues, warned it will sell crude on its own if Tripoli does not agree to its demands, the leaders said.
The government made no immediate response but has in the past threatened to destroy tankers exporting crude without its authorization.
The state National Oil Corp (NOC) cautiously welcomed the prospect of reopening ports, but warned it was too early to say what would happen. "We see this as a positive step, but have to see whether the ports will actually reopen on Dec. 15," NOC spokesman Mohamed al-Harari said.
Tripoli has refused to recognize eastern rule after the movement seized the Es-Sider, Ras Lanuf and Zueitina ports which had handled exports of almost 600,000 barrels a day.
The eastern movement is made up of fighters who helped topple Gaddafi and now demand a federal system - similar to the system in the kingdom before Gaddafi - that would share power between Cyrenaica, the west and southern Fezzan.
"Oil exports will resume (on Dec. 15) under three conditions," Ibrahim al-Jathran, an autonomy leader, told a news conference broadcast after a meeting of eastern tribes.
He repeated earlier demands for the Tripoli government to form an independent committee to investigate claims of oil sales corruption and another to group all three Libyan regions to share the oil wealth.
"The Brega (Cyrenaica in Arabic) will take its rights as guaranteed by the oil-sharing law of 1958," he said to cheering supporters.
Reuters was unable to reach Jathran, but the prime minister of his self-declared regional government, Abd-Rabbo al-Barassi, confirmed Tripoli would have to meet conditions before oil would flow again.
For its part, the oil workers' union grouping most staff said it would call on its members to shut the ports again until their pay demands are met.
"If the government does not meet our demands, then the union of oil workers will close the ports again after they reopen," deputy union head Saad al-Fakhri told Reuters.
Zeidan has been trying to get the ports reopened but has been weakened by political infighting with parliament and Islamist opponents. He announced several times that Hariga port in Tobruk would resume exporting oil, but the terminal is still in the hands of tribesmen demanding autonomy.
One problem is that the government has to deal with a mix of different protesters. Apart from Jathran's movement, a separate set of tribal leaders has blocked Hariga in the far east. Members of the Amazigh and Tibu, two minority groups, have also in the past blocked gas or oil supplies.
Barassi told Reuters the movement was ready to sell crude on its own from Dec. 15 if Tripoli did not meet the demands.
"We have the contacts with the firms to sell it," Barassi said. Another member of his self-declared government, Essam al-Jihani, also said the movement would sell crude on its own from Sunday if Tripoli did act by then.
But a third person, Salah al-Ateiwich, a leader of the Magharba tribe making up the backbone of the port blockers, said it would be difficult to sell oil bypassing Tripoli, a view shared by some oil insiders. "In my view it would be hard to sell oil to foreign companies because legally the state has a claim on it," he said, sounding optimistic Tripoli would make concessions to restart oil exports on Sunday. (Additional reporting by Ghaith Shennib; Editing by Jane Baird and Keiron Henderson)