|Southerners are confident that they will reach the 60 per cent voter threshold for the poll to be valid [Ranjit Bhaskar]
At least 11 more people have been killed in violence over southern Sudan's historic referendum, officials have said.
Major General Gier Chuang Aluong, South Sudan's interior minister, said 10 people making their way to vote were killed in an ambush on Monday.
The announcement came as South Sudan held a third day of voting on Tuesday in a referendum on whether to split from the north.
Fighters from the Misseriya tribe are thought to be behind the latest attack, which took place in Kurdufan, on the northern side of the disputed region between the north and south.
“Misseriya as a tribe belong to a country, they belong to a state, and they belong to leadership. Somebody must be responsible to take the responsibility and be accountable for what has taken place,” Aluong said, insisting the north should accept blame for the attack.
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 came four days after clashes between Misseriya nomads and southern police and youths in the contested Abyei border region, a flashpoint of north-south tensions in the past.
Observers fear the latest unrest could spark more fighting amid an otherwise peaceful independence referendum in the south.
Abyei remains the most contentious sticking point between north and south following a two-decade civil war that left some 2 million people dead.
Abyei, which holds oil deposits, had been promised its own self-determination vote. But it still remains uncertain whether it will remain part of Sudan or join an independent south.
The seven days of balloting in southern Sudan are likely to produce an overwhelming vote for independence, and Omar al-Bashir, the Sudanese president, has said he will let the oil-rich south secede peacefully.
The scale of the turnout on the first two days of the week-long poll left many southerners confident that they were well on the way to reaching the 60 per cent threshold set by a 2005 peace deal between north and south for the referendum to be valid.
The preliminary results are expected to be announced by February 7, leaving a five-day period for appeals, before announcing the final and uncontested results on February 14.
South Sudan is among the world's poorest regions and the entire region has only 50km of paved roads.
However, most of Sudan's oil is in the south, while the pipelines to the sea run through the north, tying the two regions together economically.
Bashir was also reported to have told Jimmy Carter, the former US president, that the north would take on all of Sudan's nearly $38bn debt even if the South decided to secede.
Carter, who is in the country as an international observer, said that "in a way, southern Sudan is starting with a clean sheet on debt".
However, Emad Sayed Ahmed, the Sudanese presidency spokesperson, denied this in a statement carried by the state news agency.
Bashir simply told Carter that dividing the debt burden will not be of any help to the north or the South because both sides lack the resources to make the necessary payments, Ahmed said in the statement.
Bashir's office said that trying to split the debt between the north and a possible new southern nation is of "no use" because the would-be state would not be able to service the debt.
The statement said that Sudan's debt should be scrapped altogether, adding that is was the "responsibility of the north, south and the international community".