Kenyan President Uhuru Kenyatta and Ethiopia's Prime Minister Hailemariam Desalegn have flown into Juba for talks with South Sudanese President Salva Kiir.
The leaders posed for photos before going into closed-door talks.
The visit comes amid efforts by regional powers to bring a halt to nearly two weeks of deadly unrest in the world's youngest nation.
Envoys from Kenya and Ethiopia were involved in mediation efforts last week, when their respective foreign ministers were part of a regional delegation in Juba.
Kenya has been sending in flights to evacuate its nationals from South Sudan, where many have set up businesses.
Several thousand people are believed to have been killed in a week and a half of fighting in South Sudan that has pitted troops loyal to Kiir against those backing his rival Riek Machar, a former vice-president who was sacked in July.
The unrest has taken on an ethnic dimension, pitting Kiir's Dinka tribe against Machar's Nuer.
The UN Security Council voted on Tuesday to send nearly 6000 extra soldiers and police to South Sudan, nearly doubling the United Nations Missions in Sudan (UNMIS) force to 12,500 troops and 1323 civilian police.
South Sudan gained independence in July 2011 after a bloody decades-long struggle for independence from Sudan.
UN Secretary General Ban Ki-moon has urged Kiir and Machar to negotiate an end to the violence.
In a Christmas Day radio and video message, Ban called ethnic attacks a "grave violation of human rights" and reiterated that those responsible would be held accountable.
"I want to assure you that the United Nations stands with the people of South Sudan at this difficult time.
"We are strengthening the United Nations presence and will do our best to stop the violence and help you build a better future for all," Ban said at the end of his message.
Kiir and Machar have indicated a willingness to come to the negotiating table.
