JUBA, Sudan Armed men killed 10 southern Sudanese in an ambush, a southern minister said on the third day of a referendum on independence for the south in which 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 organizers told Reuters the big turnout so far meant the total was almost guaranteed to reach the 60 percent of voters needed to make the poll valid.
"A number of returnees were ambushed yesterday by a group of armed Misseriya. They ambushed 10 buses and seven trailers loaded with the belongings with these IDPs (internally displaced persons) coming from the north," southern internal affairs minister Gier Chouang Aloung told reporters on Tuesday.
The attack took place in the northern state of Southern Kordofan close to the border with south Sudan and 10 people were killed in the attack, Aloung said.
"The 10 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.
"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 dominant southern party, the Sudan People's Liberation Movement.
Influential U.S. Senator John Kerry, who has just returned to Washington from Sudan, told Reuters he was "concerned" about the violence but did not think it would disrupt the voting.
"I don't think it will, and we're hopeful it won't," said Kerry, chair of the Senate Foreign Relations Committee. "We need to be very alert and careful about it. I think the basic leadership of both sides understands they don't want to go to war."
Previously, there were four days of confirmed clashes between Misseriya nomads and southern police and youths in the contested Abyei border region, a flashpoint of north-south tension in the past.
U.N. Secretary-General Ban Ki-moon said he was "deeply concerned" about the reports from Abyei, adding peacekeepers were stepping up patrols in the area and could be reinforced.
Thousands of people took part in the third day of voting elsewhere in the south, an undeveloped region with only 60 km (40 miles) of paved roads.
"It is proceeding very, very smoothly. There doesn't seem to be any fear of not reaching the 60 percent limit. As a matter of fact we think it will do a lot better than that," said the chair of the vote's organizing commission, Mohammed Ibrahim Khalil.
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.
The vote has unleashed strong emotions in the north where many feel bereft about the likely loss of a quarter of Sudan's land -- and the majority of its oil reserves.
In Khartoum, the home of Sudan's first leader Ismail al- Azhari was draped in black cloth in a sign of mourning.
The vote needs a 60 percent turnout to be valid. More than 50 percent of voters need to choose independence for the south to secede -- seen as the most likely outcome.
Preliminary results are expected around the beginning of February and final results before February 15. Around 4 million people signed up to vote in the south and in diaspora communities of southerners in the north and abroad.
(Additional reporting and writing by Andrew Heavens in Khartoum; editing by Tim Pearce)