, an East African regional bloc.
The United States welcomed the talks and reiterated calls for an immediate end to the fighting. National Security Council spokeswoman Caitlin Hayden said the U.S. will deny support to those who try to seize power and will "hold leaders responsible for the conduct of their forces."
The United Nations says South Sudan's violence has forced tens of thousands of civilians from their homes.
Witnesses say some of the violence is ethnically-motivated, with supporters of Mr. Kiir, a member of the Dinka tribe, and supporters of Machar, from the Nuer tribe, targeting each other for their background.
The government arrested several alleged coup plotters soon after the violence began. Hussein Mar Nyuot, a member of the rebel delegation to the peace talks, is urging the government to free political detainees.
The African Union also is urging President Kiir to free the prisoners and is threatening sanctions against those who incite violence.