Ethiopia’s foreign minister, Tedros Adhanom, said direct talks between the two sides, aimed at ending the violence, would begin on Saturday.
Fighting between supporters of President Salva Kiir and those of his sacked deputy Riek Machar has killed at least 1,000 people since 15 December.
More than 180,000 people have been displaced in the conflict. Aid workers say many are living without shelter, clean water and sanitation.
Tensions are increasing around the rebel-held cities of Bor, in Jonglei state, and Bentiu, in the northern state of Unity.
A build-up of military personnel around both cities has prompted fears that renewed heavy fighting may be imminent as the government attempts to regain control, the BBC reported.
One rebel spokesman told Reuters its troops were marching towards Juba, while a spokesman for the government said its forces were closing in to recapture Bor.
Delegates from both sides began arriving in the Ethiopian capital on Wednesday but talks were delayed until the full negotiating teams had arrived.
Observers have said the discussions are likely to be complicated, as the two sides will have to agree on a mechanism to monitor any ceasefire.
Meanwhile, the US state department said it had ordered a “further drawdown” of its embassy staff in Juba “because of the deteriorating security situation”.
It evacuated a large number of non-essential staff soon after the fighting began on 15 December.