South Sudan's President Salva Kiir has issued a decree offering a blanket amnesty to rebels, including his bitter rival Riek Machar, state radio reported Thursday.

The announcement comes days after the two men signed a power-sharing deal in the Sudanese capital Khartoum aimed at ending a nearly five-year civil war that has killed tens of thousands and uprooted millions.

"President Salva Kiir issued a presidential decree... pardoning Dr. Riek Machar and other groups that took up arms against the government since 2013. The decree comes into effect on the date 8, August 2018," State Radio Juba said, reading the decree.

Kiir also reiterated a call for his forces to observe a ceasefire agreed in June by both the government and Machar's rebels.

Sunday's power-sharing agreement paves the way for Machar to return to South Sudan as one of five vice-presidents, a role he occupied until shortly before the conflict erupted in December 2013 when Kiir accused him of plotting a coup.

A final peace accord must now be signed, under the auspices of regional bloc the Intergovernmental Authority on Development (IGAD), followed by the formation of a transitional government that will hold power for three years until elections can be organised.

The last time a peace deal was signed returning Machar to the capital was in 2015, but the agreement collapsed during days of heavy fighting in Juba after which the rebel leader fled into exile.