The secretary-general of South Sudan's ruling party and rebels signed an agreement Monday committing them to peace but the government needs 15 more days to finalise a deal, the chief mediator said.

Top party official Pagan Amum and rebel chief Riek Machar signed a deal, but mediators said Amum was not representing the government. President Salva Kiir watched the signing, after briefly shaking hands with Machar.

"They (the government) have certain reservations" and have decided to go back home for consultations, chief mediator Seyoum Mesfin said. "In the next 15 days, the president will come back to Addis Ababa and finalise the peace agreement."