South Sudan's then-rebel leader Riek Machar (R) and South Sudan's President Salva Kiir exchange signed peace agreement documents in Addis Ababa in this May 9, 2014 file photo.

South Sudan's then-rebel leader Riek Machar (R) and South Sudan's President Salva Kiir exchange signed peace agreement documents in Addis Ababa in this May 9, 2014 file photo.

© 2014 Reuters