(Releads with field reopening, adds details)
TUNIS, Aug 22 (Reuters) - Libya's Sharara oil field, the country's largest, was gradually restarting on Tuesday after repeated disruptions causing production shutdowns, two sources at the field said.
Earlier in the day, the National Oil Corporation (NOC) said a three-day pipeline blockade had ended and force majeure had been lifted on loadings of Sharara crude at Zawiya terminal. But the NOC removed its statement as an oil source said there was a further block on production at the field.
Sharara has been pumping up to 280,000 barrels per day (bpd) in recent weeks. Its production is key to a revival in Libya's oil output, which surged above 1 million bpd in late June, about four times higher than its level last summer.
Libya has been exempted from an OPEC-led push to cut global production and bolster oil prices, and the recovery of the North African country's output has complicated the bloc's efforts to curb supply.
But the renewed problems at Sharara show how fragile the gains remain, vulnerable to security challenges, local protests, political splits, and shortfalls in the NOC's maintenance and investment budget.
Sharara has been affected by repeated shutdowns because of protests by armed groups and oil workers in the field itself, along the pipeline, and in Zawiya.
Details of the second reported stoppage at the field on Tuesday were unclear, but the official said it was due to action by a different group to the one that caused a closure at a valve on a pipeline leading from Sharara to Zawiya on Saturday.
The NOC said the closure that lasted from Saturday until Tuesday morning had occurred at valve 17 on the pipeline to Zawiya. It did not say where the valve is located.
"No group has claimed responsibility for the valve closure, and no demands were made, but NOC engineers despatched to open the valve found a gearbox had been stolen," the statement said.
"This gearbox was removed by criminals for one purpose only, and that is to blockade the line in future," NOC Chairman Mustafa Sanalla was quoted as saying.
Earlier this month, production at Sharara was reduced by at least 130,000 bpd because of security breaches near the field which the NOC says have been resolved.
The NOC runs Sharara in a joint venture with oil companies Repsol, Total, OMV and Statoil . (Reporting by Ahmed Elumami, Ayman al-Warfalli and Aidan Lewis; Writing by Aidan Lewis; Editing by David Evans and Mark Potter)