Ceasefire monitors in South Sudan called on rival forces Tuesday to allow food into conflict zones where aid workers have warned tens of thousands may be dying of starvation.

"Only a fraction of the emergency food" is in place "because of restrictions on aid convoys and due to insecurity," said Festus Mogae, a former Botswana president who heads the Joint Monitoring and Evaluation Commission (JMEC), set up by regional bloc IGAD to ensure a peace deal is implemented.

Last week the UN warned that thousands of civilians have fled fighting and extreme hunger in the past month, as leaders struggle to honour a peace deal on the ground.

In October, UN-backed experts warned of a "concrete risk of famine" in parts of the northern Unity State if fighting were to continue with 30,000 people facing death by starvation outside areas aid workers can reach.

READ: Famine looms in South Sudan war zones as aid agencies blocked

Some aid has been delivered, but civilians report dire conditions and fighting has continued.

"Humanitarian projections suggest things are going to get worse before they get better," Mogae said at a meeting in Juba of the JMEC commission. "The implications are obvious."

Mogae called on both sides to order field commanders to "ensure complete and unconditional cooperation with the humanitarian agencies so that this deficiency can be remedied before it is too late."

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 country along ethnic lines.

While political efforts are ongoing in a bid to form a unity government, Mogae warned leaders they had to ensure action to create peace was made on the ground.

"The meaning of that new government will be quickly questioned if it does not rapidly address the humanitarian situation the country faces," he said.

READ: South Sudan factions agree on sharing power

Both the government and rebel sides in South Sudan's two-year conflict have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.

Fighting continues, and the conflict now involves multiple militia forces who pay little heed to paper peace deals, driven by local agendas or revenge attacks.