Arab tribesman killed 10 people and wounded 18 as they attacked a bus carrying southern Sudanese travelling from the country’s north to their home region, which is holding an independence referendum, an official said on Tuesday.

A leader of the Misseriya tribe, whose members are mostly nomadic cattle herders and which was blamed for the violence, denied there was an attack.

The south’s Minister of Internal Affairs Maj. Gen. Gier Chuang Aluong said the attack happened on Monday in South Kordofan, north of Sudan’s north-south divide. More than 140,000 southerners who have lived in the north have streamed back to the south since October - initially in order to participate in the referendum though some are now fleeing out of fear of northern retribution.

Southern Sudan’s weeklong independence referendum began on Sunday and is likely to split Sudan in two along its Arab-African faultline. The vote was part of a 2005 peace agreement that ended two decades of north-south civil war that killed two-million people. Other attacks have been reported since Friday.

The casualty figures have been in dispute, but it’s possible that around 40 people have died.

Southern officials have said that Misseriya on Sunday attacked police in the contested region of Abyei, killing 20. The tribe, for its part, accused police of killing 10 herders in about the same general area. The U.N. said it is “extremely concerned” about the reports of clashes in Abyei.

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 is to be decided in negotiations that so far have made little progress.

Voters in the south went to the polls on Tuesday for a third day of voting. Lines were considerably shorter than on Sunday and Monday, when many voters had to wait hours to cast ballots.

If southerners vote for independence, issues like north-south oil rights, water rights to the White Nile, border demarcation and the status of Abyei remain to be worked out. 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.

Successive Sudanese governments have heavily relied on tribal militias in the peripheries to quell rebellions in the country’s south and west, where Darfur is located.

The Sudanese president’s regime is accused of unleashing Arab militias known as janjaweed against rebels in Darfur. The U.N. says some 300,000 people have died in Darfur since 2003. The Sudanese government denies backing the janjaweed and says the death toll is inflated.

Sudan’s President Omar Al-Bashir is wanted by the International Criminal Court, accused of orchestrating genocide against ethnic Africans in Darfur.

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