Representatives of Kiir and rebel leader Riek Machar have been negotiating in the Ethiopian capital of Addis Ababa since August 6. The US had urged the warring factions to reach a peace agreement by August 17 or face sanctions.
Kiir and Machar will sign what he called a "compromise document" today, said Haile Michael, a spokesman for a regional bloc mediating the talks.
Several previous attempts to reach a negotiated settlement have failed. At stake were issues such as how to share power and the composition of a transitional government of national unity.