The chief ceasefire monitor in South Sudan says people are dying of starvation in the country's Western Equatoria state ​as warring sides fail to implement an agreed peace deal to end two years of war.

"I was told this morning that one of the ceasefire monitoring teams, which recently visited Mundri, found people there are starving to death," Festus Mogae, who is a former Botswana president, said on Tuesday.

"I am staggered that things have been allowed to get this bad, and I continue to urge you, the leaders of South Sudan, to do whatever you can to ensure the humanitarian effort is successful," Mogae said in a speech read to government and rebel peace delegates in Juba.

Mogae heads the Joint Monitoring and Evaluation Commission (JMEC), set up as part of a stalled August peace deal by the Intergovernmental Authority on Development (IGAD).

Last week, East Africa's IGAD regional bloc called on rival forces to allow food into conflict zones on the brink of famine, where aid workers have warned tens of thousands may be dying of starvation.

The United Nations has confirmed that thousands of people have fled recent fighting in the previously peaceful southern farming region around Mundri in Western Equatoria, close to the border with Uganda.

Both the government and rebel sides have been accused of serious violations including mass killings of ethnic groups, recruiting children and carrying out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.

Meanwhile, fighting continues between multiple militia forces driven by local agendas or revenge attacks.

South Sudan’s warring parties have missed a key deadline to form a unity government.

The rival sides were due to forge a unity government by January 22, but the rebel faction insisted that President Salva Kiir’s December decree to establish new states in the country, nearly tripling the number of regional states, undermined a key pillar of a power-sharing pact.

A spokesman for the rebel side, Mabior Garang, slammed the “anti-peace hardliners within the government,” saying the basis of negotiations should be on the old system of 10 states rather than the current 28.

South Sudan is the world's youngest country, breaking away in 2011 from Sudan after decades of war.