Sudan shut down for its first census in 15 years, a milestone in the peace deal that ended Africa's longest civil war but clouded in dispute threatening to undermine the accord further.

In the 2005 agreement signed by the former warring north and south, the two-week census is crucial to prepare constituencies for national elections and confirm or adjust the wealth and power-sharing ratios in central government.

But the undeveloped south has refused to be bound by the results and rebels in Darfur will boycott the count, both accusing the Arab north of manipulating the census to maximise its control and marginalise the African majority.

Khartoum, assisted greatly by the United Nations, says it has prepared the most comprehensive population count ever held in Sudan, almost constantly engulfed in civil war since independence from Britain in 1956.

"The planning and field work in the south has been the best possible... They have every enumerator in place and (we have) the international resources to get the best possible census," said Yasin Haj Abdin, director of the central bureau of statistics.

Around 60,000 enumerators, monitored by 200 observers, will count the estimated 40 million population, costing Sudan and the international community 103 million dollars.

The central bureau of statistics expects census results as early as September, but other officials have quoted Christmas as a more realistic date.