Sudan and South Sudan are still supporting rebels in each other's country as they prepare for a fresh round of talks this week over disputes that brought the two neighbors to the brink of war earlier this year.
The relationship between South Sudan and the United States, in particular, has become strained by reports of the south's support for the rebels despite repeated American objections.
Sudan's military is fighting the rebels, known as the Sudan People's Liberation Movement-North (SPLM-N), in the states of South Kordofan and Blue Nile. Hundreds of thousands of people have been displaced in the year-old conflict, and the border region faces a humanitarian crisis.
Dozens of Sundanese troops and five rebels died in weekend fighting in South Kordofan. The SPLM-N said its fighters killed 61 soldiers.
The rebels are a vestige of an earlier rebellion led by the Sudan People's Liberation Movement, which for 22 years fought a civil war against government forces that left more than 2 million dead. That conflict ended with a peace deal in 2005 and the independence of South Sudan on July 9 last year. Yet peace has been elusive between the African neighbors.
The Obama administration wants the Sudanese government to sign on to an African Union proposal that would demilitarize the border and facilitate monitoring of support from South Sudan for the rebels.
"If the government of Sudan would agree to the [African Union] map for demilitarization of the border, much of any such support to the SPLM-N from South Sudan could be monitored and reported to an investigative committee," said Princeton Lyman, the U.S. special envoy for the two countries.
"It is hard to understand why they would not want to do so, if they feel that support to the SPLM-N from South Sudan endangers their security," he added.
Sudan's government opposed the AU plan because the document includes a map that puts a 14-mile area of disputed territory within South Sudan's borders, said Al-Obeid Murawih, a spokesman for the Sudanese Foreign Ministry in a phone interview from the capital, Khartoum.
He said Sudan would accept the AU plan if the map is corrected.
"We are open to any solutions," he added.
Talks between Sudan and South Sudan are scheduled in the Ethiopian capital, Addis Ababa, on Thursday. The rebel group fighting in South Kordofan and Blue Nile also will participate.
Support for the rebels has turned into a prickly issue between U.S. officials and their South Sudanese counterparts.
South Sudanese President Salva Kiir has insisted that his government cut off all ties to the group after his country became independent. The rebels get their arms from Sudanese forces they defeat in battle, he said on a visit to Washington in December.
Yet months after Mr. Kiir made these assertions, Western activists returning from the region said there was ample evidence of links between South Sudan's army and the rebels.
Mr. Lyman said U.S. officials discuss this matter with their South Sudanese counterparts on an "ongoing basis."
"[South Sudan] has said that it has long-standing links with the SPLM-N," he added. "We believe that there will not be a secure border, nor satisfactory resolution of any cross-border aid, until there is a political settlement in [South Kordofan and Blue Nile] and a demilitarized and monitored border."
Sudanese and South Sudanese officials accuse each other of financing and arming rebels, but they deny doing so themselves.
"The [southerners fighting] in South Kordofan and Blue Nile are part and parcel" of South Sudan's army, Mr. Murawih said.
"They are under the authority of President Salva Kiir," he added.
South Sudanese Information Minister Barnaba Marial Benjamin dismissed the accusations.
"We are not giving any support," Mr. Benjamin said in a phone interview from South Sudan's capital, Juba.
"Our president has discussed that with [Sudanese] President [Omar] Bashir and has said we will assist [him] if he is involved in a peaceful dialogue in trying to get a political solution," he added.
A Western activist who was in the border region between Sudan and South Sudan earlier this year said there are "solid ties" between South Sudan's army and the rebels fighting in Sudan.
"Everybody recognizes that there is a relationship between the two," the activist said, speaking on background in order to freely discuss sensitive security issues.
Camps have been set up in the South Sudanese border states of Unity and Upper Nile for refugees fleeing the fighting. The South Sudanese army and rebel fighters have set up checkpoints near these camps. Army personnel are present outside the Jamam refugee camp in South Sudan's Upper Nile state, while the rebels have set up checkpoints at Al-Fuj border crossing.
A pay dispute
Heated disputes between the rebels and South Sudanese army officers over salaries were reported earlier this year.
"They are not co-mingling their duties, but the payroll dispute shows that the linkages still remain," said the Western activist. "While the [South Sudanese army] leadership doesn't want to claim any formal relationship, everybody realizes that there is a very close relationship between the two. They haven't formally divorced."
The rebels in Sudan are still unofficially referred to as Division 9 and Division 10 of South Sudan's army in South Kordofan and Blue Nile, respectively.
After South Sudan became independent, these two divisions were supposed to return to the south, but they stayed in what they saw as their own jurisdiction.
"Military and logistical cooperation and collaboration between the two forces continues," said the Small Arms Survey, an independent research project located at the Graduate Institute of International and Development Studies in Geneva, in an April report, debunking claims that the two forces had separated.
"Both Khartoum and Juba [South Sudan's capital] have raised the stakes by supporting allies to destabilize the other," the report said. "In most cases, mutual accusations of military and logistical support are legitimate, while the ongoing denials have little credibility."
Jonah Leff, project coordinator for the Small Arms Survey's Sudan/South Sudan Project, said: "We gathered pretty credible testimonies."
The rebels in South Kordofan seized most of their weapons from the Sudanese military, but there probably is a steady flow of ammunition coming from South Sudan, he added.
Anwar Elhaj, a Washington-based spokesman for the SPLM-N, denied his group receives financial or material support from South Sudan.
He describes the relationship his group has with South Sudan as historical.
"We still share the same vision, but financially and militarily we are separate entities," he said. "But this is an internal issue that has to do with Sudan, not South Sudan."
The southern rebels have joined forces with groups in Sudan's western state of Darfur. Their goal is to overthrow the government of Gen. Bashir, who is wanted by the International Criminal Court for war crimes and crimes against humanity in Darfur.
Sudan, too, is supporting rebels fighting in South Sudan.
The Small Arms Survey report found that Sudan is the primary source of arms for those rebels.
A review of weapons used by rebels in South Sudan showed that a large number of arms, mostly Chinese-made, had been provided by Sudan, Mr. Leff said.
"[The rebels] are all in Khartoum," said Mr. Benjamin, the South Sudanese official. "Everybody knows they are still supporting them."
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