Armed men killed ten southern Sudanese in an ambush, a southern minister said on the third day of a referendum on independence for the south, but voters have defied gloomy predictions and turned out in huge numbers.

The attack on a convoy of people returning to the south for the referendum was the latest reported violent incident to mar the week-long vote, which is expected to see the south emerge as a new nation.

Vote organisers told Reuters a big turnout so far was almost guaranteed to reach the 60% of voters needed to make the poll valid.

"A number of returnees were ambushed yesterday by a group of armed Misseriya. They ambushed ten buses and seven trailers loaded with the belongings with these internally displaced persons (IDPs) coming from the north," southern internal affairs minister Gier Chouang Aloung told reporters on Tuesday.

Aloung said the attack had happened on the northern side of the border between the northern state of Southern Kordofan and the southern state of Northern Bahr al-Ghazal and local authorities had told him ten died in the attack.

"The ten south Sudanese could have voted ... These attacks are not in south Sudan. It is in northern Sudan. The Misseriya is not a foreign tribe. It is in Sudan ... so the north is responsible."

Mohamed Wad Abuk, a senior member of the area's Arab Misseriya nomads, denied any involvement in the attack.

"This is a lie and the Misseriya has not attacked any convoy. The SPLM just want to exploit the situation in the area to create confusion," he said, referring to the south's dominant party the Sudan People's Liberation Movement.

The latest attack comes after four days of confirmed clashes between Misseriya nomads and southern police and youths in the contested Abyei border region, a flashpoint of north-south tensions in the past.

Thousands of people took part in the third day of voting elsewhere in the south, an undeveloped region with 60 km (40 miles) of paved roads, the second highest rates of maternal mortality in the world and one of the worst records for primary school attendance.

"It is proceeding very, very smoothly. There doesn't seem to be any fear of not reaching the 60% limit. As a matter of fact we think it will do a lot better than that," said the chairperson of the vote's organising commission, Mohammed Ibrahim Khalil.

Khalil, a northern lawyer based in Khartoum, told Reuters some polling centres had already received between a quarter and a half of the voters registered in their district in the first two days of the week-long vote.

The referendum was promised in the 2005 Comprehensive Peace Agreement that ended Africa's longest civil war between the mostly Muslim north and the south, where most follow Christianity and traditional beliefs.

Under the regulations, the vote needs a 60% turnout to be valid. More than 50% of voters need to choose independence for the south to secede –seen as the most likely outcome.

Final results are due out before February 15 with preliminary figures expected up to two weeks earlier. Around 4 million people signed up to vote in the south and in diaspora communities of southerners in the north and abroad.