Sudan has delayed the registration of voters for January's referendum on secession for the south until November, raising tensions over the timetable.
The chairman of the referendum commission said this was to allow for staff training and delivery of forms.
Tanzania's former President Benjamin Mkapa, appointed by the UN to encourage a smooth vote, has told the BBC many challenges lie ahead.
But he said if all parties were willing, the timetable would be met.
The referendum was part of a 2005 peace deal to end two decades of conflict between the north and oil-rich south in which some 1.5 million people died.
Analysts fear there is a risk of the conflict restarting if southerners feel that Khartoum is trying to delay or disrupt the vote in the oil-rich region - one of the world's poorest and least developed regions.
Chairman of the Southern Sudan Referendum Commission, Muhammad Ibrahim Khalil, said the registration had been delayed by three weeks till 15 November.
According to Reuters news agency, registration forms have not yet arrived from the printers in South Africa - and are not due until late October.
Mr Mkapa was appointed last week by UN Secretary General Ban Ki-moon to lead a special monitoring panel to ensure a smooth build up to the polls.
Despite the panel's deployment, Sudan's government in Khartoum remains overall responsibility for the referendum, the UN says.
Asked on the BBC's Network Africa programme whether he would suggest a delay to the vote if preparations did not go to plan, Mr Mkapa said: "We'll cross that bridge when we get to it.
"But as of now I feel our obligation is to make sure it happens on the 9th of January."