S. Sudan peace deal at 'critical' stage: African Union

South Sudan's repeatedly broken peace deal is at a "critical stage", the African Union has warned, calling on rival forces to honour commitments amid fears it is close to collapse.

AU Commission chief Nkosazana Dlamini-Zuma warned that all sides must "abide by the commitments they have entered into under the peace agreement, including the permanent ceasefire."

Internationally backed ceasefire monitors of the August 26 deal held their first meeting Friday in Juba -- but with battles raging, rebels failed to turn up.

File picture shows South Sudan's soldiers siting on a truck at the airport in Juba ©Samir Bol (AFP/File)

Dlamini-Zuma, in a statement released late Saturday, said the rebels must "ensure that they attend subsequent meetings" of the Joint Monitoring and Evaluation Commission (JMEC), set up by East Africa's IGAD bloc.

The United States, Britain and Norway, key backers of the peace process, have warned the deal will "unravel" if the rivals continue to miss deadlines.

"Each day, the fighting and abuses continue, and an already grave humanitarian situation grows worse," a joint statement Wednesday read.

Civil war began in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.

"The end of the war has already been declared numerous times: let us now make it a reality," JMEC chief Festus Mogae said Friday.

"There is a need for a paradigm shift, from war to peace, from destruction to reconstruction," the former president of Botswana said.

The commission is meant to include representatives of both sides, and Mogae warned it will work only if it is a "collaborative" body.

The conflict has triggered a humanitarian crisis with 2.3 million people forced from their homes and 4.6 million in need of emergency food. Tens of thousands have died and the economy is in ruins.

In October, UN-backed experts warned of a "concrete risk of famine" before the end of the year if fighting continues.

Both sides are accused of having perpetrated ethnic massacres, recruited and killed children and carried out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.

President Kiir met with Pope Francis in Uganda on Friday, but no details of the brief meeting have emerged.