File image - A South Sudanese soldier looks at warplanes as he lies on the ground to take cover beside a road. REUTERS/Goran Tomasevic

JUBA - Sudan's simmering border rebellions could yet unravel a freshly signed deal with South Sudan and jeopardise the expected resumption of the South's oil flows through Sudan, a British envoy said on Friday.

Sudan and South Sudan, long-time foes, agreed last week on a timetable to create a demilitarised buffer zone along their contested 2,000-km (1,250-mile) border, capping months of acrimonious negotiations.

That move to secure the shared boundary cleared the way for landlocked South Sudan to order oil companies to resume pumping its crude oil through Sudan to the Red Sea, ending a 15-month shutdown that had hit both economies.

Diplomats who brokered the deal had to overcome deep distrust between both sides - and Khartoum's repeated accusations that Juba was supplying weapons to rebels fighting in the Sudanese border states of Blue Nile and South Kordofan.

“A lot of (the deal) risks being undermined if Sudan believes that South Sudan is continuing to support what they see as their rebels,” said Alastair McPhail, the British Ambassador to South Sudan.

“They don't have to be good friends, they just have to be good neighbours.”

Relations between Sudan and South Sudan remain deeply troubled following decades of fighting fueled by oil, ethnicity, religion and territorial disputes.

The 1983-2005 civil war between Khartoum and southern rebels ended in a peace deal that paved the way for the South to declare independence from Sudan in July 2011.

But border skirmishes brought both countries' armies close to war in April last year - and Sudan has continued to clash with the border state rebels, many of whom sided with the south during the civil war and say they want to overthrow the Khartoum government.

“If there is no agreement in Southern Kordofan and Blue Nile there will always be tension because the border is so important at the moment,” McPhail, who is leaving South Sudan later this month, told Reuters in an interview.

Sudan's repeated accusations that the South is backing the insurgents have scuppered past efforts to get an accord. Khartoum walked away from a deal in September, demanding guarantees that the support had stopped.

Juba rejects the charge and accuses Sudan of supporting insurgents in the South.

South Sudan's decision to shut down its oil flow months after declaring independence took away its only real source of hard currency and devastated its already impoverished economy.

McPhail said the state needed to do more to improve its legal system if it wanted to have any chance of attracting more foreign investment.

Several foreign businesses have been forced to leave because they have been unable protect their investments, he added.

“(This) sends a very poor message to the international community. I think it's a lot of isolated cases. It's not just one or two but I'm not sure that it's endemic,” he said.

McPhail said he is also worried about the South Sudan government's increasing intolerance of dissenting voices.

Several critics have fled the country in recent months. The government denies it is clamping down on critics, and says it is investigating shooting death of an outspoken columnist in December.

“It impacts on all of us. Not just the media but diplomats have been harassed. Foreign nationals have been harassed,” McPhail said.

“It's about a pattern of behaviour and if some people in the organised forces are going beyond their mandates, if they are killing people extra-judicially, if they are detaining people, then that's a concern not just to us but to South Sudanese.” - Reuters