President Salva Kiir and rebel chief Riek Machar have repeatedly accused each other of breaking successive peace deals but say they remain committed to the August 26 agreement, despite missing every listed deadline.
Yesterday, an advance team of rebel delegates arrived in Juba, tasked with beginning the formation of a transitional government of national unity - but did not include Machar.
"There is scepticism in many quarters that peace has really come, and there are understandable doubts, that can only be addressed by meaningful action," said Festus Mogae, who heads the Joint Monitoring and Evaluation Commission (JMEC), set up by the regional IGAD-bloc to ensure the peace deal is implemented.
Civil war began in December 2013 when Kiir accused his former deputy Machar of planning a coup, setting off a cycle of retaliatory killings that have split the poverty-stricken country along ethnic lines.
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Delegates at an earlier meeting held a minute's silence for the tens of thousands killed since war broke out in December 2013.
"Let that not be an empty gesture for the cameras," Mogae said, a former president of Botswana.
"It is my sincere hope that this Christmas, 2015, is the last celebrated in a context of conflict," said Mogae.
Both sides are accused of having perpetrated ethnic massacres, recruited and killed children and carried out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.
The conflict has triggered a humanitarian crisis with 2.3 million people forced from their homes and 4.6 million in need of emergency food. Tens of thousands have died and the economy is in ruins.