JUBA - South Sudan is not withdrawing troops from the border with Sudan to set up a buffer zone as it pledged it was last month, South Sudan's army said on Monday, in a setback to efforts to resume the oil exports vital to both economies.The two countries came close to war last April in the worst border clashes since South Sudan seceded in 2011 under a peace deal that ended one of Africa's longest civil wars.The African Union managed to broker a deal in September to defuse hostilities. But the nations have failed set up a buffer border zone and resume oil exports from the landlocked South Sudan through Sudanese pipelines as agreed in Addis Ababa.In a sign of goodwill, South Sudan said three weeks ago it had started to unilaterally withdraw its troops from the border and would set up its side of the 10-km buffer zone by Feb 4.Such a buffer zone is a pre-condition for Sudan to allow oil exports to restart. Juba shut down its output of 350,000 barrels day a year ago in a row with Khartoum over pipeline fees.But South Sudan's military spokesman Philip Aguer said on Monday the army had not even started to pull out from the border, despite the government statement."There are no orders to withdraw and I don't think there will be any unless there is an agreement from both governments," Aguer said. "We will never withdraw unless there is an agreement for a timely withdrawal for both armies."Neither South Sudanese nor Sudanese government officials were immediately available for comment.The African Union twice brought together Sudan's President Omar Hassan al-Bashir and his South Sudanese counterpart Salva Kiir in Ethiopia last month to end the stalemate but there has been no sign of progress.Over the weekend Juba accused Khartoum of killing a South Sudanese soldier in two air attacks on their border. Sudan denied the allegation, though Reuters reporters have witnessed Sudanese air attacks on southern territory in the past.