Wednesday’s meeting in Ethiopia’s capital, Addis Ababa, comes amid efforts to end the world’s youngest country’s five-year civil war, which has killed tens of thousands of people and caused about four million to flee their homes.
The warring leaders will meet at the invitation of Ethiopia’s Prime Minister Ahmed Abiy.
Ateny Wek Ateny, South Sudan’s government spokesman, called Abiy’s invitation “significant”, telling The Associated Press news agency: “anything that brings peace in South Sudan is wanted.”
The meeting was initially scheduled for Tuesday but postponed by one day at the last minute. It will be mediated by the Intergovernmental Authority on Development (IGAD), an East African regional bloc that has led several rounds of failed peace talks.
Having split from Sudan in 2011, South Sudan descended into civil war in 2013 after Kiir accused Machar, then his deputy, of plotting a coup.
Since then, multiple ceasefire agreements and peace efforts have failed to yield any positive results.
Machar fled South Sudan in July 2016 after new fighting erupted in the capital, Juba, ending a brief attempt at peace in which he returned to his role as Kiir’s vice president.
The rivals have not met since.
Pressure has been growing on the warring sides to end a conflict that has created Africa’s largest refugee crisis since the 1994 Rwanda genocide.
Both sides have been accused of abuses against civilians, including along ethnic lines.
Earlier this month, the UN Security Council adopted a US-sponsored resolution that threatens an arms embargo on South Sudan and sanctions against six individuals, including the country’s defence chief, if fighting does not stop and a political agreement reached.
The resolution asks UN Secretary-General Antonio Guterres to report to the Council on that by June 30.