Libya is shutting down production at its biggest oilfield after tribesmen and state-paid security guards seized the El Sharara facility, threatening oil workers, several field engineers and protesters said on Sunday.
If confirmed by state oil firm NOC, the shutdown at the 315,000 barrel-per-day field would also force the closure of the El-Feel oilfield, located in the same remote and lawless southern desert.
El-Feel usually pumps around 70,000 bpd.
"El Sharara is closed," one engineer said. A spokesman for the tribesmen which call themselves the Fezzan Anger Movement, Mohamed Maighal, also said the field was shut.
NOC confirmed only that guards had forced the closure of some pumps at El Sharara "which will result in on-site tanks filling within the next few hours and thus force a production shutdown," a statement said.
NOC could not be reached for further comment but said it would issue another statement later. It warned of "catastrophic consequences".
"Shutting down production at the El Sharara field will have catastrophic, long-term consequences, it would take a long time to resume production because of the sabotage and theft that are likely to happen," NOC said in a statement.
The tribesmen stormed into the field premises on Saturday after NOC said some guards, supported by locals, had opened the gates, driving around in jeeps and filming themselves in video they sent to journalists.
They stayed overnight in the vast, partly unsecured area, making good on a threat to stop production issued first in October should authorities not provide more development funds for their impoverished region.
"We will not allow the El Sharara field to reopen unless the U.N. mediates," Maighal said late Sunday afternoon.
He said their southern Fezzan region had suffered decades of neglect and demanded that the revenue of the oil produced from local fields be used to fund development projects.