Kenyan President Uhuru Kenyatta and Ethiopia's Prime Minister Hailemariam Desalegn flew into Juba on Thursday for talks with South Sudanese President Salva Kiir, an AFP journalist said.
The leaders posed for photos before going into closed door talks. The meeting was confirmed by both the Ethiopian foreign ministry and Kenyatta's office.
The visit comes amid ongoing efforts by regional powers to bring a halt to nearly two weeks of unrest in the world's youngest nation.
Envoys from both Kenya and Ethiopia were already involved in mediation efforts last week, when their respective foreign ministers were part of a regional delegation that went to Juba.
Kenyan Foreign Minister Amina Mohammed accompanied Kenyatta to Juba, her office said.
Kenya has been sending in flights to evacuate its nationals from South Sudan, where many have set up businesses.
Several thousand people are now 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.
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The unrest has taken on an ethnic dimension, pitting Kiir's Dinka tribe against Machar's Nuer.
The United Nations on Thursday said it hopes to begin receiving critical reinforcements of military hardware and personnel within the next 48 hours for its overstretched peacekeeping mission in South Sudan.
Hilde Johnson, head of the UN mission in South Sudan, known as UNMISS, told reporters that some 50,000 civilians were seeking protection at UN bases across Africa's youngest country, which gained independence from Khartoum in 2011.
"We are working around the clock to get assets in that can assist us in the current crisis as quickly as ever possible, and we have had conversations with other (UN) missions today," she told reporters by video link from Juba. "We are working on 48-hour delivery of several of the critical assets."
Johnson declined to elaborate on what those assets were, though she said they included both personnel and hardware. UN officials have said that the South Sudan peacekeeping mission needs transport helicopters and planes, as well as troops.
Johnson said that her mission would not abandon South Sudan, vowing, "We are here to stay."
"The scale of this crisis has challenged an already overstretched mission," she added.
The UN Security Council voted Tuesday to send nearly 6,000 extra soldiers and police to South Sudan, nearly doubling the UNMISS force to 12,500 troops and 1,323 civilian police.
South Sudan gained independence in July 2011 after a bloody decades-long struggle for independence from Sudan.
Agence France-Presse and Reuters contributed to this report.