(Pete Muller ~ Associated Press)
Separate clashes in the disputed border region of Abyei were also reported, but officials from the south and north gave varying accounts of the fighting, ranging from one wounded to nine dead.
The weeklong referendum begins Sunday and is likely to see Africa's largest country split in two. In order for the referendum to pass, a simple majority must vote for independence and 60 percent of the 3.9 million registered voters must cast ballots.
Southern Sudan President Salva Kiir, who met with U.S. Sen. John Kerry on Saturday, addressed southern voters at a news conference attended by a mass of international media.
"I urge you all to make your decision in a peaceful manner as we end the longest journey," Kiir said. "From our side as the government of Southern Sudan, we promise you an atmosphere of calm and guaranteed security."
Kerry, who is one of several high-profile American observers in southern Sudan for the vote, said the stability of northern Sudan is also critical. Sudan will lose a third of its land, nearly a quarter of its population and much of its main moneymaker, oil, if south Sudanese vote for independence.
"It's not only a moment of self determination for the south, if they decide to do that, it's also the renewal of the nation in the north ... That's our hope, whether it happens or not is up to its own leaders," Kerry said.
The attacks against the Sudan People's Liberation Army happened Friday and Saturday in an oil-rich area bordering northern Sudan. In the southern capital of Juba, though, residents were making preparations for the momentous vote.
"Why is it happening now? The intention or the motive behind must be undermining the referendum," said Gen. Acuil Tito Madut, the inspector general of the south's police. "Once these fights break out in these states that will mean some people will not vote and once people don't vote that means the required percentage is not achieved."
Southern army spokesman Col. Philip Aguer said forces loyal to rebel leader Gatluak Gai attacked SPLA forces overnight Friday and into Saturday in Unity State, an oil-rich area bordering northern Sudan. Aguer said six rebels died in the exchanges.
Jonglei state, meanwhile, saw deadly clashes between men commanded by militia leader David Yauyau and the southern military, said Madut. One civilian was among those killed, Madut said.
Madut said 32 rebels from Gai's group were captured by the southern military and were being brought to Juba, the southern capital, to be interrogated about who is behind the group. The men were captured with 30 AK-47 assault rifles, one machine gun and one rocket-propelled grenade, he said.
In the disputed area of Abyei, a southern official, Charles Abyei, said that an Arab tribe known as the Misseriya attacked black southerners Friday and Saturday, killing between seven and 10. A member of the Misseriya tribe, Abdul-Rassoul Al-Nur, though, said southern security forces attacked Misseriya cattle herders, wounding one Friday.
Abyei had been due to hold a separate referendum Sunday to decide if it would be part of the north or the south, but now the future of the region looks like it will be decided by north-south negotiations.
Sudan President Omar Al-Bashir told Al-Jazeera TV on Friday that resolving the Abyei issue might take time and each side must work to control their people. He said the north would control the Misseriya unless the southern Dinka tribe "make a first move. Then there will be a reaction."
Southern Sudan suffered through decades of internal strife while it was at war with the north. It has worked in recent weeks to strike peace deals with armed southern dissidents, but Gai's and Yauyau's rebels have not yet reconciled.
Starting Sunday, southern Sudanese will cast simple, illustrated ballots at polling stations under thatched roof shelters in the remote and impoverished countryside and in Juba, a city of simple concrete houses and mud huts that got its first paved roads only in recent years.
If it passes, the referendum will split Africa's biggest country between the mostly Arab and Muslim north, and the mostly black and Christian or animist south. Southern Sudan would then be on track to become the world's newest country in July. Outstanding issues like sharing oil wealth, water rights and demarcating the border still have to be agreed to.
Aid groups also fear that southerners living in the north and northerners living in the south will face harassment and abuse.
The U.S. has made the referendum a foreign policy priority and has offered to remove Sudan from the list of state sponsors of terror if Khartoum doesn't hinder the vote. Actor George Clooney, a Sudan activist, and former President Jimmy Carter are also in the country for the vote.
Sudan's 1983-2005 civil war killed an estimated 2 million people and left many others missing one or more limbs.
"What we are doing is something that we have not done in our lifetime. This is something that has never happened," said Justice Chan Reec Madut, the top southern official in the referendum commission. "Nobody every bothered to ask the people of Southern Sudan as to what their destiny should be."
Associated Press reporter Sarah El Deeb in Khartoum, Sudan contributed to this report.