South Sudan President Salva Kiir and rebel chief Riek Machar are set to meet Tuesday in a fresh push to end nearly six months of civil war, sources said Monday.

A source close to the talks in the Ethiopian capital said the rivals are set to meet "in the afternoon" on Tuesday, along with regional leaders from the East African bloc IGAD.

A face-to-face meeting between the foes "would be very good" for the peace process, the source told AFP.

The summit would be the first time leaders of the warring parties have met since signing a ceasefire on May 9, the second since January.

But the ceasefire has been repeatedly broken with fighting continuing in the young nation, deepening a crisis which has already killed thousands and forced over 1.3 million from their homes.

Information Minister Michael Makuei said in a statement the government was "optimistic that this round of discussions will sow the seeds of peace".

Meanwhile Human Rights Watch urged mediators to not allow any amnesty to be included in a peace deal, and to punish those guilty of war crimes.

"The mediators need to break with the past by ensuring that amnesty for serious crimes is not on the table," HRW Africa director Daniel Bekele said in a statement.

The United Nations has accused both sides of possible war crimes, and has called on both sides to respect peace deals.

Aid agencies are also warning of a deepening humanitarian crisis and the risk of famine should the war continue, while a cholera outbreak has spread beyond the capital Juba.

The conflict has pitted government troops against forces loyal to Machar. The violence has taken on an ethnic dimension, with Dinka people loosely tied to Kiir fighting the Nuer, Machar's tribe.