* Neighbours struck deal in September to resume oil exports
* Both said was necessary to create border buffer zone
* Have been unable to agree how to withdraw armies
A top South Sudanese official arrived in Sudan on Saturday to discuss how to set up a demilitarised border zone, a condition for resuming oil exports, in the first direct talks between the neighbours since new tensions broke out last month.
The African countries agreed at talks in Ethiopia in September to end hostilities and restart oil exports - including creating the buffer zone - after coming close to war in April, the worst violence since South Sudan seceded last year.
South Sudan had shut down its oil production of 350,000 barrels a day in January after tensions over pipeline fees escalated.
But the neighbours have been unable to agree how to withdraw their armies from the disputed border, a step both had said was necessary to resume oil exports from landlocked South Sudan through Sudanese pipelines.
"I came here from Juba to activate the joint cooperation agreements signed between the two countries in Addis Ababa for the benefit of the two people," Pagan Amum, South Sudan's chief negotiator, told reporters at Khartoum airport.
He said he had brought a letter from South Sudan's President Salva Kiir for his Sudanese counterpart Omar Hassan al-Bashir.
Sudan's top negotiator Idris Abdel-Qadir said it was in the interest of both countries to break the deadlock.
"We welcome the visit of our brother Pagan in Khartoum and, as our brother Pagan said, the aim of his visit is to implement the cooperation agreements," he said.
Security officials from both countries will meet from Monday in Khartoum to discuss setting up the demilitarised zone.
On Monday, Kiir accused Sudan of putting new obstacles in the way by demanding that South Sudan needed first to disarm rebels fighting the Khartoum government inside Sudanese territory.
Sudan has not publicly responded to the comments but has accused South Sudan of supporting rebels of the Sudan People's Liberation Movement-North (SPLM-North), which operate in two states bordering South Sudan.
South Sudan denies backing the SPLM-North, which seeks together with rebels from the western region of Darfur to topple Bashir.
The new tensions in the past two weeks have delayed resuming oil production in South Sudan that had been originally scheduled for Nov. 15, a serious blow to both crumbling economies.
South Sudan became independent in July 2011 after a referendum agreed under a 2005 peace agreement which ended decades of civil war between the Muslim north and the South, where most follow Christian and African faiths.
South Sudan inherited three-quarters of Sudan's oil production when it seceded but needs to pay Khartoum for using northern export pipelines to the Red Sea coast.