LONDON, April 6 (Reuters) - Sudan risks returning to war unless factions in the south receive swift aid to rebuild lives shattered by Africa's longest civil confict, a key U.N. official says.

SPLA soldiers train at military headquarters.

Sudan will ask international donors at a meeting in Oslo next week for $2.6 billion (1.4 billion pounds) to develop vital infrastructure following a peace deal ending 21 years of fighting between Khartoum and southern rebels.

"The message is that unless you put money on the table Sudan risks returning to war," said Jon Bennett, U.N. team leader for Sudan's Joint Assessment Mission, which has drawn up a plan for the country's reconstruction.

Under January's peace deal Khartoum and the former rebel Sudan People's Liberation Movement (SPLM) will set up a coalition government, decentralise power, share oil revenues and form joint military units.

Although the SPLM is the dominant southern group, Bennett said there were plenty of factions which could feel they had reason to resume fighting over resources in marginalised areas.

"Tribal chiefs will say you may have (a peace agreement), but where's my hospital and where's my school. And unless they see something they will start blaming the SPLM," he said.

"It's really very urgent. This whole thing could be derailed."

Bennett said chiefs in the kingdom of Shilluk could be tempted to war against the Dinkas, who dominate the SPLM -- particularly because the area may have oil.

Violence has already flared since the peace deal south of the oil rich area of Bentiu and in Akobo on the border of Ethiopia.

"The SPLM needs to pacify southern groups in its area," he said.

"Unless they see a real dividend they will go back to localised war.

"There is tension and from time to time you hear of flare ups ... people have not seen a reason to support the peace process. They have not seen something tangible."

NO TARMAC ROADS

Bennett was speaking ahead of a donors' meeting in Oslo on April 11-12 where the development plan for Sudan will be presented.

But Bennett said before the south could begin building hospitals or schools the first task was to create an administration.

"They have never been ministers of even politicians before so they need very basic training in administering an area which is the size of France and Germany together.

"There's no point in building a hospital today because there are no doctors. And there's no point in building schools because there are no teachers. That's the dilemma in the south."

Other priorities include new roads -- there is no tarmac road in the whole of the south -- and demining so that people can start cultivating fields.

But Bennett said there were grounds to be optimistic.

"The level of development in the south is one of the worst, if not the worst in Africa. Yet the per capita wealth is probably one of the highest.

"There are 10 million people and half the oil wealth of Sudan."

Sudan reckons it will need $7.8 billion between July 2005 and the end of 2007 to build roads, railways, hospitals and schools. But around two thirds of that will be funded from revenues from its oil production of about 320,000 barrels per day.

More than 2 million people were killed and 4 million displaced by the civil war which pitted the mainly animist and Christian south against the Arab north. The war was complicated by issues of oil, ethnicity and ideology.