A boy from South Sudan carries a container of water at a railway station refugee camp in Khartoum. (Reuters)
The United Nations warned Tuesday it had received just two percent of the cash needed for South Sudan, a day after experts said 40,000 people are being starved to death.
Some war-zone areas are on the brink of famine.
"If we are unable to act now, the situation will be much worse," UN aid chief in South Sudan Eugene Owusu said.
"About two percent of the $1.3 billion (1.1 billion euros) required to provide life-saving assistance and protection has been received".
On Monday, the UN children's agency UNICEF and the World Food Programme (WFP), in a joint statement, described some of the worst conditions yet in more than two years of a civil war marked by atrocities and accusations of war crimes, including the blockading of food supplies.
Over 2.8 million people need aid, almost a quarter of the country, while in war zone northern areas 40,00 are being starved to death with aid blocked amid violence.
For South Sudan, the beginning of the year is "when the country is traditionally most food secure," the UN added.
"We are facing widespread food insecurity, malnutrition, displacement and disease," Owusu said, pleading for an initial $200 million to fill gaps until May. "We are facing increasing needs with diminishing resources."
The UN-backed specialised hunger assessment, the Integrated Food Security Phase Classification (IPC) report, warned in October of a "concrete risk of famine" without aid.
On Monday, it said areas are most concern were too dangerous for experts to access, meaning it was not possible to gather the statistics needed to declare famine.
Civil war erupted 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.