South Sudan faces possible famine if warring factions do not heed a ceasefire deal they have signed, US and EU envoys warned Thursday.

"The parties have failed to respect their commitment to the January 23 cessation of hostilities," said US envoy Donald Booth, at a meeting of regional leaders in Ethiopia on the conflict in the world's youngest country.

"This failure has led to thousands of additional deaths and an ever worsening humanitarian situation, with South Sudan facing a possible famine," he added.

Over 930,000 civilians have fled their homes since fighting began in mid-December, including over quarter of million leaving for neighbouring nations as refugees, according to the United Nations.

"If things continue as they are, the spectre of famine looms," EU envoy Alexander Rondos said.

"The rains are coming, and if people cannot go and plant there will not be sufficient stocks within months... This should be a matter of the greatest urgency."

Heavy fighting has continued despite the January deal, with the army on Thursday reporting continued clashes in the strategic oil city of Malakal, one of the hardest-fought battlegrounds in the conflict.

Both the US and EU envoys were speaking at a special one-day summit in Addis Ababa of leaders from the East African bloc, the Intergovernmental Authority on Development (IGAD).

Leaders pleaded for the two sides to end the fighting.

"I strongly urge the warring parties to immediately cease hostilities as per their agreement," Kenyan President Uhuru Kenyatta said.

"Neither IGAD's appeal for peace nor ours is about telling the parties what to do," Booth added. "It is about appealing to them to do the right thing."

Ethiopian Prime Minister Hailemariam Desalegn, who hosted the talks, warned that "negotiations are about readiness to give and take," as he opened the summit, which was attended by Kiir.

"We should say 'no' to spoilers who are not interested in peace," Hailemariam said.

(AFP)

File photo: A refugee and child in a South Sudan camp, 2012 (MSF/Nichole Sobecki)