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 accusations of war crimes, including the blockading of food supplies.

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

Those worst affected are in the northern battleground state of Unity, once the country's key oil producing region, but now scene of some of the heaviest fighting, including the mass abduction and rape of women and children.

"Unless unrestricted humanitarian access is urgently granted... food insecurity could deteriorate to famine in parts of Unity state," the statement added.

Famine is a technical measure, assessed by the Integrated Food Security Phase Classification (IPC), which classifies hunger on a scale of one to five.

While large parts of South Sudan's Unity and Upper Nile regions were already classified as being just one step short of famine, termed "Emergency" or "phase four", areas in Unity have been declared to be in "phase five" for the first time.

Level five is classified as "Catastrophe", and when stretched to 20 per cent of the population, becomes famine.

"Since the war in South Sudan started nearly two years ago, it is the first time that an IPC analysis has found any parts of the population in phase five, 'catastrophe'," the UN added.

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.

"Since fighting broke out nearly two years ago, children have been plagued by conflict, disease, fear and hunger," Unicef chief in South Sudan Jonathan Veitch said.

"Their families have been extraordinary in trying to sustain them, but have now exhausted all coping mechanisms. Agencies can support, but only if we have unrestricted access. If we do not, many children may die."

A year ago famine was averted only after a huge intervention by aid agencies.