The Unites States has warned President Salva Kiir and opposition leader, Machar, not to violate the peace deal aimed at ending a brutal civil war in the world's youngest state.

The US affirmed that it would "hold to account" leaders who breached the deal, and would support sanctions against them, said state department spokesman John Kirby as it did not recognise President Salva Kiir's reservations about the deal.

President Kiir signed the deal on Wednesday. Rebel leader Riek Machar signed it last week.

Fighting between forces loyal to the two men over the last 20 months has forced more than 2.2 million people from their homes in South Sudan, which broke away from Sudan in 2011.

The deal envisages Mr Machar returning to the government as vice-president.