Thousands had gathered under the shade of the trees by the airstrip in Akobo, waiting for the arrival of a United Nations helicopter packed with food aid.

Bony-armed women in ragged clothes held children with distended bellies, slack skin hanging off tiny limbs, their hair turned orange by severe malnutrition. Most stared blankly ahead, too weak from hunger to cry.

Every person here has fled bloody inter-tribal violence. More than 2,000 people have been killed by fighting in southern Sudan so far this year — more than in the war-torn western region of Darfur. More than a quarter of a million have been displaced in three southern states alone. South Sudan is now in the grip of a humanitarian crisis, and one and a half million people do not have enough food, with the UN warning that it is on the brink of a famine.

“The situation is really bad; we haven’t had food for over two months,” said Nyayual, who fled her village after it was attacked by a rival tribe. “We’ve been surviving by eating grass.” Nyayual and her family were attacked again when her community was forced to leave Akobo to hunt for food. They were ambushed on a fishing trip, and more than 180 people were killed. Nyayual lost seven family members, including two children.

The fighting has been concentrated in Jonglei state, a vast marshland cut off from the rest of the country for more than half the year by flooding. Tribal skirmishes over cattle and grazing land are common and seasonal, and there are always a few fatalities. The authorities are struggling to understand the increase in the number of killings.

There has also been a disturbing new trend, with women and children directly attacked. Nearly all of those killed in the attack against Nyayual’s community were women and children. “They must have planned it for when the men had left the camp. They came after us one by one. We had no-one to defend us,” said Nyayual.

The violence comes at a critical time for Sudan. The country’s first multiparty elections in two decades will be held next year and voter registration is already under way. A referendum for independence for the mostly Christian and animist South from the mainly Muslim North is set for January 2011. Preparations for the elections and referendum have been delayed by disagreements between the North and South on laws governing the two votes, fuelling existing tension.

History of trouble

1955 First civil war between North and South begins

1956 Independence. In the years that follow, Sudanese politics is characterised by military coups

1978 Oil reserves are discovered in Bentiu in southern Sudan

1983 Southern-based Sudanese People’s Liberation Army (SPLA) is formed to fight government forces

1999 Sudan begins to export oil

2002 Government and SPLA sign peace deal bringing 19 years of civil war to an end

2003 Fighting breaks out in the Western region of Darfur. Pro-government Arab militia are accused of carrying out ethnic cleansing of non-Arab civilians

2005 A comprehensive peace deal is signed between Government and southern rebels. The South forms an autonomous Government made up mostly of rebels. The UN says that those who commit attrocities in Darfur can be brought before the International Criminal Court

2007 Sudan accepts the deployment of a 26,000-strong UN force to Darfur

2008 North and South begin intense fighting over the oil-rich town of Abyei

July 2009 North and South accept a ruling in The Hague that gives control of the Abyei region and its oilfields to the North

Source: BBC, Times database

Unreported World: How to Fuel a Famine, Friday November 6 at 7.35pm, Channel 4.

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