Nairobi - Weeks after the UN-backed experts warned that thousands are dying of starvation in South Sudan war zones, aid agencies say they cannot access areas to stave off famine because of "spiralling" violence.
Last month, the UN agencies said 30 000 people were starving to death in Unity state, with Integrated Food Security Phase Classification (IPC) experts warning of a "concrete risk of famine" before the end of the year if fighting continues and aid does not reach the hardest hit areas.
Despite the warnings and an August peace deal, there has been no let-up in government and rebel battles, with diplomats accusing both sides of blockading life-saving aid.
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Medical aid agency Doctors Without Borders (MSF) warns that the "humanitarian crisis in southern Unity is on an unprecedented scale" with "repeated and targeted violence" against civilians.
MSF, which has earned a reputation for operating in some of the world's most dangerous frontline battle zones, said it had "not seen this level of violence and brutality before."
While an official famine has not been declared, the IPC report describes the worst conditions yet seen in a nearly two-year civil war marked by atrocities and accusations of war crimes, including gang rape and forced cannibalism.
Having been forced to flee their homes, many people are living in the open, said Lindsay Hamsik, policy advisor at South Sudan NGO Forum, an alliance of over 300 South Sudanese and international aid agencies.
"The commitments at the political level are not being held up on the ground," Hamsik told AFP. "That means more lives are being lost and civilians continue to be terrorised and denied their right to assistance."
'Abductions, mass rapes and killings'
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.
"All parties urgently need to lift obstacles that prevent access of humanitarian organisations," the United States and European Union warned last week, in a statement also signed by diplomats from Canada, Norway and Switzerland.
"This is crucial to avoid the looming famine."
Most aid agencies were forced to pull out in May amid intense fighting.
"Our efforts to re-secure access have failed," said Caelin Briggs from the Norwegian Refugee Council (NRC). "While we are eager to return, and we will take any window that we can, we cannot do so unless the violence comes to a halt."
Aid workers said they feared that without boots on the ground, they simply may not know the full extent of the crisis.
The International Rescue Committee (IRC) said people were "hiding in the swamps and bushes" and that they were struggling with the "limited availability of evidence".
Some 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.
'It will get worse'
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 that have split the poverty-stricken, landlocked country along ethnic lines.
The army and rebels have repeatedly accused each other of breaking an internationally-brokered August 26 ceasefire, the eighth such agreement aimed at ending the nearly two-year long war.
"MSF teams in Unity state hear daily reports of extortions, abductions, mass rapes and killings, and witnessed villages burnt to the ground and crops looted and destroyed," MSF added.
Hardest hit are the counties of Leer, Guit, Koch and Mayendit in Unity State. MSF's base in Leer was looted on October 3, forcing MSF to close the hospital for the second time since May.
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Leer, hometown of rebel chief Machar, has swapped hands several times in the war.
In the latest violence, ceasefire monitors from the East African IGAD bloc blamed government forces who they said "looted medical supplies and other equipment" when they pushed out rebels in Leer in early October.
"Violence against the civilian population is escalating," MSF emergency manager Tara Newell said. Before staff were forced to evacuate, dozens of children depended on specialised feeding to survive.
With no aid, "those children who were already identified to be severely malnourished are very likely to have died," Newell added.
Aid workers are gloomy for the future.
"People have exhausted the last of their coping mechanisms," Hamsik said. "It's already a humanitarian crisis, and it will get worse."