Counting has begun in Sudan's first national census since 1993, a required step in the country's peace process. But as Derek Kilner reports from Nairobi, former rebels governing semi-autonomous southern Sudan continue to express concerns about the exercise, and have reserved the right to reject the results.

Sudan's two decade, north-south civil war left the country without a reliable population count, particularly in the southern region that bore the brunt of the fighting.

A national census is a necessary first step before conducting elections in 2009 and a referendum on southern secession in 2011, and is also needed to determine ratios for power-sharing and wealth-sharing between the north and south during the current interim period.

The exercise, funded jointly by the Sudanese government and the United Nations, was supposed to start in January, but has been repeatedly delayed. Most recently, the Sudan People's Liberation Movement, former rebels now running the semi-autonomous southern government - delayed last week's start date because of concerns the census would not properly count southerners.

The Sudan People's Liberation Movement is concerned that many of the hundreds of thousands of southerners who were displaced or fled across the border to escape fighting have not been able to return home and will not be counted.

The former rebels say since religion and ethnicity will not be recorded, the many displaced southerners living at the north could be counted as northerners.

Southerners, largely Christian and non-Arab, say they have been undercounted in previous censuses and hope to show that Sudan is not the mostly Muslim and Arab country suggested by available statistics.

The Sudan People's Liberation Movement now says the census will proceed, but reserves the right to reject the results if they are not seen as adequately reflecting the country's diversity.

Sudan program director, Hafiz Mohammed, of London's Justice Africa organization, acknowledges such obstacles, saying the country is not prepared for a proper count. But he says the Sudan People's Liberation Movement should not have waited until the last minute to try to halt the census.

"They have to have a plan. They have to know what they are actually doing," he said. "To just come in the week before the census and say no, we cannot not hold it because of that reason. These reasons were there for two or three years, they are not new."

Mohammed says the southern government should share the blame for the current obstacles, not having done enough to facilitate the resettlement of refugees and internally displaced people.

The United Nations and the international community have pushed to avoid delays in the census, given its important role in maintaining the timetable of the 2005 north-south peace agreement. But under the current circumstances, the census may end up causing more problems than it solves.

Mohammed warns that rejection of the results by the southern government would seriously jeopardize the peace agreement.

"If you are accusing one party of manipulating that means you are not going to have a free and fair election," he said. "And the whole Comprehensive Peace Agreement will be in serious trouble, because you must have the election by 2009. And if that does not happen the question is what is going to happen to the government of Sudan."

The obstacles could be even greater in the western region of Darfur, where a significant portion of the population will likely not be counted due to widespread displacement and continued conflict. All of the major rebel groups in Darfur have rejected the census, with some threatening to attack those attempting to carry it out.

Results are expected sometime after September.