Rebel forces in South Sudan threaten the crucial oil fields key to the economy, experts warn, risking possible military intervention by old enemies Sudan should production not be protected.

On Saturday, a key army commander in control of Unity state -- one of the most important petroleum regions -- defected to join the rebel forces of Riek Machar, the fugitive former vice president.

Army spokesman Philip Aguer insists that troops loyal to President Salva Kiir remain in control of Unity state -- including the oil fields -- and that it is only the state capital Bentiu that has fallen.

But as oil workers flee, the loss of the state capital in a region awash with guns and a long history of rebellion is a major blow.

"The potential for oil wealth to exacerbate the current power struggle should not be underestimated," said Emma Vickers of Global Witness, an international campaign group.

"If rebel forces were to capture the oil fields, they could effectively hold the government to ransom."

South Sudan survived a year-long shutdown of oil production during a fee dispute with Sudan, the land-locked South's only access point for markets. But it is still reeling from the loss of billions of petro-dollars.

Oil accounts for more than 95 percent of the economy, and the country would struggle should production be shut down again.

Oil companies have already been evacuating workers.

Chinese state oil company China National Petroleum Corp (CNPC) has confirmed it is pulling out its staff.

Hundreds of foreign oil workers, including nationals from China and Pakistan, have been queueing at Juba's airport waiting for the first flight out, meaning production is at best hugely curtailed.

"The worst-case scenario is rapidly unfolding: political and personal disputes are escalating into an all-out civil war in which certain ethnic groups are increasingly targeted by the others' forces and the rebels take over the oilfields," wrote John Prendergast, longtime analyst and campaigner on the region, in a joint article with actor and activist George Clooney.

"Khartoum's government has long exploited divisions within South Sudan and provided support to various armed groups to sow further division and destruction."

Old enemy Sudan fears oil cut

Sudan has already said it fears for the fate of vital oil flows.

Sudan's cash-starved economy is to receive an estimated $1.5 billion (1.1 billion euros) in fees from South Sudan next year for moving crude through northern pipelines for export.

"The worst scenario for us is this war flares in other areas and extends to the whole of the South, and this will have an impact on Sudan," Information Minister Ahmed Bilal Osman said Friday.

Control of the oil fields will be a key aim of all those fighting, he warned.

"Definitely one of the targets of the two powers, will (be to) try to take over the oil fields," perhaps as a way to improve their bargaining position, he told AFP.

"It's a struggle for wealth and power."

During the 1983-2005 Sudanese civil war, Khartoum backed militia forces to protect oil fields against the southern rebel army, which now -- after the South's independence in 2011 -- is the official military.

Several of those oil-field militia forces were integrated into South Sudan's army, but with army factions now mutineering, some may have turned to rebellion.

After intermittent border fighting last year, and allegations that the South was supporting rebels in Sudan, relations improved with a September summit between Kiir and Sudan's President Omar al-Bashir.

The two leaders agreed to implement a raft of security and economic pacts.

But those deals "will be very difficult to implement if you don't have a partner in Juba," a foreign diplomat said, asking for anonymity.

"Sudan could take advantage of the deepening internal conflicts in Juba, despite previous assurances that any deals between the countries will not be impacted by political crises in South Sudan," said Ahmed Soliman, of Britain's Chatham House think thank.

Sudan itself is grappling with insurgencies in the Kordofan area and Blue Nile state that border the South.

"Certainly the Sudan regime might see the instability in the oilfields as an opportunity to aggressively move into bordering regions, take possession of some of the southern oil areas, and keep the oil flowing northward," said Prendergast and Clooney.

While Sudan is involved in regional peace efforts aimed at kickstarting talks between the warring sides, foreign diplomats in Khartoum say there is little Sudan can do to stop the violence.

Any intervention risks worsening the volatile situation.

"They should resist any temptation -- if there is any -- to get involved," said another foreign diplomat in Khartoum. "It can only end in disaster."

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