A polling centre official explains to a Southern Sudanese woman the two voting signs, separation and unity, on the referendum ballot, on the third day of voting in the city of Um Durman, Sudan. (Nasser Nasser/Associated Press)

More than 60 per cent of registered voters have already cast ballots in Southern Sudan's weeklong independence referendum, crossing the threshold needed for the vote to be valid, a southern official said Wednesday.

The south's secession would split Africa's largest country in two. Sudanese President Omar al-Bashir has said he will let the south go peacefully if it votes to split from the north.

Ann Itto, an official with Southern Sudan's ruling Sudan Peoples' Liberation Movement, told journalists on Wednesday that nearly 2.3 million voters had cast ballots so far, surpassing the 60 per cent of registered votes needed to ensure the outcome's validity. Nearly four million people registered to vote.

Some two million people died in a two-decade war between north and south Sudan that ended in 2005 with a peace agreement that allowed for the referendum on independence. The weeklong vote has been jubilant, though the future of the desperately poor region remains uncertain.

Sudan (CBC)

Because only 15 per cent of Southern Sudan's 8.7 million people can read, the ballot choices were a drawing of a single hand marked "separation" or another of clasped hands marked "unity."

Southerners, who mainly define themselves as African, have long resented their underdevelopment, accusing the northern Arab-dominated government in Khartoum of taking their oil revenues without investing in the south. Southerners — mainly animists or Christians — were also angered by attempts of the northern dominated government to impose Islamic law.

Independence won't be finalized until July, and many issues are yet to be worked out. They include north-south oil rights, water rights to the White Nile, border demarcation and the status of the contested region of Abyei, a north-south border region where the biggest threat of a return to conflict exists.

Violence in Abyei has left at least 30 people dead along Sudan's north-south divide in recent days, officials have said.