|Southerners are confident that they will reach the 60 per cent voter threshold for the poll to be valid [Ranjit Bhaskar]
Voting has opened for the third day in southern Sudan's historic referendum to determine whether the region will secede from Sudan to become an independent state.
Jubilant voters had flooded polling stations for a second day on Monday, but tensions remained high in the border region of Abyei with reports of at least 30 people being killed in violence over the past few days.
Observers fear the latest unrest could spark more fighting amid an otherwise peaceful and jubilant 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.
Martin Nesirky, a United Nations spokesman, said on Monday that the organisation is "extremely concerned" about the reports of clashes and casualties in Abyei.
"The mission is in the process of confirming the numbers (of casualties), and containing the situation with enhanced patrols and engaging with the top leadership," Nesirky said.
Abyei, which holds oil deposits, had been promised its own self-determination vote, but now whether it remains part of Sudan or joins an independent south will be decided in negotiations that so far have made little progress.
Meanwhile, the South Sudan Referendum Commission said that at least 20 per cent of southern voters cast their ballots in the poll on secession.
"The percentage of those who voted [on Sunday] in the northern states was 14 and in the southern states it was 20 percent," Paulino Wanawilla Unango, of the South Sudan Referendum Commission, said on Monday.
More than 60 per cent of all registered voters must vote for the referendum to be considered valid.
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 commission said it decided to extend voting by an hour to give the voters more time to cast their ballots.
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.
The seven days of balloting 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.
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".