South Sudan's enemy within
Even before South Sudan declares its independence next week, it is already fighting at least half a dozen rebel movements.
On a video recording obtained by the BBC, hundreds of southern fighters jog rhythmically in a wide circle, singing and flaunting their new weapons.
The apparently joyous scenes in the video clash violently with a bloody reality: The rebel groups have fought on numerous occasions with the southern army, and represent a great threat to the stability of the new state.
The motivations of the rebels vary, but most of their leaders are former senior officers in the southern army, the Sudan People's Liberation Army (SPLA), or militia leaders who fought with Sudan's government during the 21-year year civil war, which ended in a peace deal paving the way for the south's independence.
One of the rebel groups, Peter Gadet's South Sudan Liberation Army (SSLA), says it is fighting corruption, lack of development, and the domination of the Dinka ethnic group.
The Dinkas form South Sudan's biggest ethnic group, and are accused of holding most of the key position in the southern army and government.
Spending on the SPLA accounts for more than a quarter of South Sudan's budget, and is about three times as much as the money spent on health and education combined.
In part, this is a recognition of the threat posed by the rebel groups and the old enemies in Khartoum.
But most of the money goes on salaries, and the senior UN official in South Sudan, David Gressley, recently said the army should be halved after independence.
The SSLA has fought a series of battles with the SPLA, near its bases in Unity state.
According to the SPLA, all the groups have one thing in common: They are funded and supplied by the former enemy in Khartoum.
The video of the rebels was given to the BBC by a rebel leader who was, at least temporarily, in Khartoum.
Rightly or wrongly, Juba sees the hand of Khartoum in every fresh mutiny, with serious consequences for relations between the two.
During the two-decade-long civil war, the north made a habit of funding rival groups to weaken the SPLA.
Even South Sudan's Vice-President Riek Machar once split off from the SPLA.
President Omar al-Bashir's National Congress Party (NCP) denies it is supporting a new generation of rebel groups.
"Peter Gadet and the others were not part of the NCP or the Sudan Armed Forces, they were ex-SPLA militia, who abandoned them following the rigging of the elections in the south," says senior NCP official Ibrahim Ghandour.
"This is a south-south business, and the north and the NCP are not implicated in this."
In the video obtained by the BBC, the SSLA troops seem to be wearing brand new boots and uniforms, and are equipped with weapons which show no signs of wear.
They proudly display mortars, machine-guns and rocket-propelled grenades.
None of the rebel groups have been able to satisfactorily explain how they got this new weaponry.
There are hints they will put their shiny new guns to work at or just after the celebrations of South Sudan's independence.
But how big a threat do they actually pose?
The various groups - by some counts close to a dozen - talk about co-ordinating their actions, but would probably find it difficult due to South Sudan's limited infrastructures and large size, not to mention the tricky question of who should lead.
All the same, the UN estimates 1,400 civilians have died so far this year in South Sudan, due to government or rebel activity, or inter-ethnic conflict.
Some of those deaths are blamed on the SPLA, a rebel movement itself once, and one that has struggled to make the transition to a national army.
The SSLA, at least, announced its intention, in the Mayom declaration, to overthrow the government.
Some observers have their doubts.
"The rebel groups are not a threat to the government in Juba, but are a local source of instability," says EJ Hogendoorn of the think-tank International Crisis Group (ICG).
"This can trigger humanitarian crises in those areas and undermine development."
This matters in South Sudan, which by some reckonings will become the least developed country on earth when it declares its independence.
"One of the most devastating impacts of the resurgence of violence in Unity [state] is the re-mining of roads cleared of war-time mines by UN and international demining groups since 2005," the Small Arms Survey pointed out in a recent report.
If they are unlikely to overthrow the state, the rebels may well have other objectives.
SSLA spokesman Bol Gatkouth Kol hints his troops could seize control of the oil fields in Unity state.
That would have vast repercussions, since oil represents about 98% of South Sudan's revenue.
Another possibility is senior commanders could convert military success into a lucrative return into the SPLA.
"The SPLA is extraordinary," comments one long-time observer of Sudan.
"Officers seem to see going into rebellion as a way of jumping up a few rungs on the career ladder."
Still, all the rebels say they have legitimate concerns.
The perceived predominance of Dinkas in the top echelons of the army and its related party, the SPLM, and the SPLM's reluctance to open the political process up to other parties, are certainly part of them.
"There are many complex grievances in South Sudan," says Mr Hogendoorn.
"Many of them were suppressed in the interest of South Sudan's independence, but they are beginning to surface as 9 July approaches.
"Unless the SPLM becomes more open and governs more inclusively, these grievances will fester and lead to more rebellion."
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.