What you need to know:
- What started as a power struggle between South Sudan's President Salva Kiir and his former deputy Riek Machar in late December quickly split the army and exacerbated ethnic feelings
- Cash-strapped South Sudan is embroiled in a war that has displaced almost a million people, with many wondering how long the baby nation can last
As ongoing fighting in South Sudan shatters any pretence of a ceasefire between government troops and rebels, analysts fear the conflict could engulf the region, as former foes fight old wars in a new country.
There are tensions between South Sudan's northern neighbour and old enemy Sudan and its new ally Uganda, while chief mediator Ethiopia is likely alarmed by allegations that its arch-enemy Eritrea is funnelling weapons from ally Khartoum to South Sudan's rebels.
One Western diplomat, speaking on condition of anonymity, said that the worst case-scenario currently being discussed is that "you've got Uganda fighting Sudan inside South Sudan, with Eritrea fighting Ethiopia inside South Sudan and a complete law and order vacuum."
"As far as the regionalisation of the conflict goes, the question is not if, but when", said Casie Copeland, South Sudan analyst for the Brussels-based International Crisis Group think tank.
What started as a power struggle between South Sudan's President Salva Kiir and his former deputy Riek Machar in late December quickly split the army and exacerbated ethnic feelings in a nation born less than three years ago, after five decades of war with Khartoum.
A feud between Ugandan President Yoweri Museveni and his Sudanese counterpart Omar al-Bashir is rooted in Uganda's rescue of a then southern guerrilla army battling Khartoum forces in the last stages of the civil war that ended with a 2005 peace deal and South Sudan's independence six years later.
Uganda has long accused Khartoum of funding rebel groups such as the Lord's Resistance Army on its territory.
With Ugandan troops now openly backing Kiir and pushing north, and as rebels retake territory near oil fields whose produce flows north for export, Sudan could soon go on the offensive.
"We do worry about the regionalisation of this. The Ugandans and Sudanese hate each other," the diplomat said.
Rebels from Sudan's troubled Darfur region, who have been battling Khartoum for a decade, are also reported to be fighting alongside South Sudan in oil-rich Unity state, and ethnic militia from other parts of the south are also pitching in, further straining north-south relations.
There are real fears that Sudan could revert to old tactics of funding proxies or arming the opposition as calls for Uganda to withdraw go unheeded and oil assets are threatened.
A row in early 2012 over how much South Sudan should pay to export its oil through northern pipelines to port led to an 18-month halt in production that brought both economies to their knees.
Kiir and Bashir eventually signed deals on oil and security that included a promise to stop funding one another's enemies to ensure their co-dependent stability.
The current conflict has called this and also the future of South Sudan's oil revenues into question, with many seeing newly oil-rich Uganda's military support as a bargaining chip for oil.
South Sudan's defence minister recently admitted that Ugandan troops were on the payroll, contradicting his colleagues' claims there were no foreign forces in country.
But speculation is rife over what else South Sudan's government -- still reeling from the shutdown -- has offered to secure its position.
"I heard one of the oil blocks, or so much (oil) per day", said the diplomat.
South Sudan's decision to build an alternative pipeline to Kenya or Djibouti further unnerved Khartoum and raised Uganda's hopes it might link up to an East African pipeline and build its own refinery.
With French oil giant Total owning concessions in Ethiopia, Uganda and South Sudan, "the pieces are there for big play", says Copeland.
RISK OF BANKRUPTCY
But for now, cash-strapped South Sudan is embroiled in a war that has displaced almost a million people, with many wondering how long the baby nation can last.
"International financial experts are suggesting that the government might be bankrupt in 2-3 months," said one analyst on condition of anonymity.
South Sudan's biggest oil-backer China has played a role in supporting the government financially with development projects and loans, and is now increasingly wading into internal politics.
But its weight is unlikely to cut through strained regional relations as paranoia over history repeating itself with regards to oil, guns and borders grows.
"A nightmare scenario is unfolding in this region", said John Prendergast in a recent report for the anti-genocide Enough project.
He has called for a proper investigation into whether Eritrea is providing arms through Sudan and for an extension of sanctions already incurred by Asmara for supplying weapons to Somali groups.
Until there is no "visible presence" of Ugandan forces in South Sudan, he doubts that peace talks in Ethiopia's capital Addis Ababa that the host and Sudan have backed will ever start in earnest.
But even without confused regional backers, half-hearted international pressure or China's economic carrot, left alone, South Sudan's war could rumble on for years, funded solely by the spoils of corruption and determination of former guerrilla leaders.