Nairobi - Over 30 000 people in South Sudan's war-zone regions face death by starvation, the United Nations said on 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.

At least 3.9 million people are in crisis - a third of the country's population - a massive 80% rise compared to the same period last year, the UN said.

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

'Likely to deteriorate into famine'

An IPC report released Thursday warned that without "urgent and immediate humanitarian access", the situation was "likely to deteriorate into famine".

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% 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.

Conditions on the ground for affected individual households are therefore already effectively in famine conditions.

While poor rains have impacted harvests in some areas, the worst conditions are in war-zone areas, with the extreme conditions sparked by conflict not climate.

Hardest hit are the counties of Leer, Guit, Koch and Mayendit, areas where aid agencies have been forced to pull out in recent months due to intense fighting.

"People are on the edge of a catastrophe that can be prevented," said WFP chief Joyce Luma.

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.

'Many children may die'

"Since fighting broke out nearly two years ago, children have been plagued by conflict, disease, fear and hunger," the 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."

Tens of thousands of civilians have been killed from fighting and the country's economy has been destroyed, with soaring inflation causing sharp spikes in food prices.

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

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 across the country that has split the poverty-stricken, landlocked country along ethnic lines.

The army and rebels have repeatedly traded blame, accusing each other of breaking an internationally-brokered August 26 ceasefire, the eighth such agreement.