Juba, Sudan, Jan 08 (Agencies): A rebel group attacked Southern Sudan's military and four rebels were killed, a military spokesman said on Saturday, a day before the region begins voting in an independence referendum that is expected to divide Africa's largest nation in two.

Despite the attack, most officials predict the weeklong vote to go peacefully.

Col Philip Aguer said forces loyal to rebel leader Gatluak Gai attacked forces in the Sudan Peoples' Liberation Army possibly on early Saturday, in Unity State, an oil-rich area on the north-south Sudan divide.

He added the counterattack by SPLA forces killed four and wounded six of Gai's men.

Southern Sudan suffered through decades of internal strife and civil war with the north. It has worked in recent weeks to strike peace deals with dissident commanders, but Gai's rebels have not yet reconciled with the southern Government.

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.