NASIR – Thick lines of sweat run down the face of Peter Gatwech as he clutches the dressing around the bullet hole in his belly.

"I was hit by the guns of the soldiers when we were fighting," said the 24-year old cattle keeper from the Jikany branch of the Nuer people, his voice quivering with pain.

The young man is one of 33 wounded from the latest round of vicious fighting in southern Sudan who have received treatment in the hospital in Nasir, an impoverished town of mud and thatch huts in Upper Nile state.

"They were sending supplies to the Luo, and we had to stop them," he added, referring to a rival Nuer people whose lands border Jikany territory.

Gatwech was with several hundred armed Jikany men who launched an attack in mid-June on a river convoy of 30 barges carrying United Nations food aid, killing at least 40 of the 150 southern soldiers acting as its escort.

At least three boats were sunk and over 700 tonnes of grain and other supplies for the UN's World Food Programme were looted.

Ethnic clashes are common in the south, a remote area awash with automatic weapons from Sudan's 22-year long civil war that only ended in 2005.

Some are sparked by cattle rustling and disputes over natural resources.

But the scale of violence and the increasing number of attacks on women and children is causing increasing concern.

In all, more than a thousand have died and many thousands more have been displaced by fighting in the south in recent months, with UN officials warning that the recent rate of violent deaths now surpasses those in Sudan's war-torn western region of Darfur.

In the mid-June incident, the food was intended for a group of 19,000 people in the isolated region of Akobo who had fled a separate outbreak of fighting.

But the Jikany assert that arms were being smuggled to the Luo in separate boats following the UN convoy and they could not allow supplies to pass to a people who attacked them last month, massacring 71 in the village of Torkech.

"The Luo surrounded the village when we were asleep," said Nyakem Jok, a teenage girl from Torkech, still recovering in hospital from the gunshot blast to her leg.

"Some of us were asleep outside under mosquito nets, others inside the huts, when they started firing from all sides," Jok added.

The hardworking but overstretched hospital in Nasir, run by the aid agency Medicine Sans Frontieres (Doctors Without Borders) Holland, treated 54 from Torkech for bullet wounds.

"It was a particularly tragic incident: only three were men, and the youngest child was only two-months-old," said surgeon Sebastian Lawrenz.

"That number of injured people, all arriving within a few hours, is an enormous amount for a Western hospital to cope with, let alone for the basic setting we have here."

The bitter Jikany-Luo battles, though terrible, are rooted in local grievances.

However, southern leaders are claiming old rivalries across the region are being deliberately provoked to destabilise the south ahead of elections in February and a historic independence referendum due in 2011.