When not plotting military strategy to seize South Sudan's crucial oil fields, sacked vice-president turned rebel chief Riek Machar spends time reading the economic and political history "Why Nations Fail".

Cynics might argue he would do better to simply look around his basic bush camp, where mutinous soldiers and an allied ethnic militia crammed with child soldiers ready themselves to attack government forces, as a brutal four-month-long civil war in which thousands of people have already been killed intensifies.

"I didn't want to fight any more war again," Machar told AFP in a recent interview at his rebel hideout, saying people had had enough of fighting during Sudan's long civil war, in which he was a guerrilla commander.

It was that war, which lasted more two decades, that paved the way for South Sudan's independence from the north.

But although less than three years old, the world's youngest nation is spiralling towards collapse. With a ceasefire deal in tatters, the United Nations fear more than one million people are at risk of famine, and analysts warn the war is dragging in regional nations.

Over one million people have fled their homes, with violence worsening amid a renewed offensive by the rebel forces, as well as revenge attacks by multiple militia forces.

Peace talks in luxury hotels in Ethiopia have made little if any progress, while analysts warn that any solution will require major changes, not simply more promises inked only on paper.

"Propping up the government in Juba and polishing its legitimacy with a dose of political dialogue and a dash of power sharing will not end the conflict," the International Crisis Group (ICG) wrote in a recent report.

- UN peacekeepers 'outgunned' -

On Thursday hundreds of gunmen stormed a UN peacekeeping base in the flashpoint town of Bor, killing at least 48 men, women and children sheltering there from a rival ethnic group before peacekeepers fought them off.

The UN Security Council called the attack an "outrage" that may constitute a war crime.

"Badly outgunned peacekeepers are no match for the thousands of heavily armed forces and militias," the ICG added.

When fighting broke out on December 15, it was sparked by "primarily political" arguments between Machar and President Salva Kiir, the ICG said, but the battles have since escalated, spreading to other states in the oil-rich but grossly impoverished nation.

"Ethnic targeting, communal mobilisation and spiralling violence quickly led to appalling levels of brutality against civilians," according to the ICG.

Atrocities were also carried out further north in the oil-hub of Bentiu, which the army admitted on Wednesday it had lost to rebel forces.

The UN aid agency said it had reports of "targeted killings based on ethnicity", with "several dozen" corpses rotting on the streets.

The violence is rooted in decades-old grievances between former rebels turned political leaders, combined with unhealed wounds left over from the long civil war that preceded South Sudan's independence from Khartoum in 2011.

The fighting is between soldiers loyal to Kiir against mutinous troops who sided with Machar, but has also taken on an ethnic dimension, pitting Kiir's Dinka tribe against militia forces from Machar's Nuer people.

- 'Worse yet to come' -

Many of the fragile gains made by the billions of dollars of international development aid that poured in after independence have been lost.

"The war risks tearing the country further apart and is pulling in regional states," the ICG said, pointing to a plan by regional nations to send in military forces in addition to UN peacekeepers.

Neighbouring Uganda has sent in troops and fighter jets to back the government, while Information Minister Michael Makuei has accused "forces from Sudan" of backing Machar, although he stopped short of actively accusing the government in Khartoum of interfering.

Back at the camp Machar predicts, gloomily, that "this will be a regional conflict".

He says he is "looking for funding" but rejects accusations that he is seeking support from neighbouring Sudan, old friends who backed him during the 1983-2005 war.

Rival gunmen from Sudan's war-torn Darfur are accused of fighting on both sides in South Sudan.

"Worse is yet to come," Jonathan Veitch, the UNICEF chief in South Sudan said last week, warning if the war is not stopped, there will be "child malnutrition on a scale never before experienced here."

The United States, the key backer of South Sudan's move to independence, has threatened targeted sanctions.

Experts say sanctions would be symbolic, but they fear they would have little positive impact.

"Many ordinary people seem to think that it is about time world powers spoke up against the absurdity of this war," said Jok Madut Jok, a former top government official who is now head of the Sudd Institute think tank.

But he also said he fears sanctions would mean little to rebels stationed in the remote bush, while the government could be pushed "into further rogue behaviour, having nothing more to lose."