Over 30,000 people in South Sudan's war-zone regions face death by starvation, the United Nations said Thursday, warning that "tens of thousands" more are on the "brink of famine."

While an official famine has not been declared, the report describes the worst conditions yet seen in a 22-month civil war marked by atrocities and accusation of war crimes.

"At least 30,000 people are living in extreme conditions and are facing starvation and death," the UN's Food and Agriculture Organization (FAO), the UN children's agency UNICEF and the World Food Programme (WFP) said in a joint statement.