People in southern Sudan are due to start voting in a landmark referendum on independence.
The week-long vote is widely expected to result in Africa's largest country being split in two.
Ahead of the referendum, the region's leader Salva Kiir said there was no alternative to peaceful co-existence between the north and the south.
The poll was part of the terms of the 2005 peace agreement which ended the war between the two.
Speaking in the grounds of the presidential palace in Juba, Mr Kiir said the referendum was "not the end of the journey but rather the beginning of a new one".
"There is no substitute for peaceful coexistence," he added.
He was speaking alongside US Senator John Kerry, who has been in dialogue with both northern and southern leaders attempting to smooth the process ahead of the vote.
As the vote neared, six people died in an attack by rebels on southern Sudan's military in the oil-rich area of Unity state.
Col Philip Aguer, a military spokesman, told the Associated Press that his troops had retaliated and killed four of the rebels.
UN officials confirmed that they had received reports of an attack in the area, but did not say which side had suffered the fatalities. BBC News