Nearly two million South Sudanese have fled, and tens of thousands have been killed, since the country descended into civil war nearly four years ago.
In May, the United Nations said it needed $1.4bn this year alone to help people who have sought refuge in Uganda, Sudan, Ethiopia, Kenya, the Democratic Republic of Congo and the Central African Republic.
But on Monday a senior UN official told AFP that so far only 30% of this amount had been raised.
«The shortfall is 70% and we are already in the middle of the year,» said Arnauld Akodjenou, UN regional coordinator on South Sudanese refugees.
He said the crisis had been further aggravated by «famine and diseases» in parts of the world’s youngest country.
Aid groups have denounced a «man-made» famine caused by the conflict in South Sudan that has also disrupted agriculture, sent prices soaring and cut off aid agencies from some of the worst-hit areas.
South Sudan gained independence from Sudan in 2011, and the civil war erupted in December 2013 in a power struggle between President Salva Kiir and his former deputy Riek Machar.
Akodjenou said there were fears that aid agencies might even have to cut food aid to refugees if the funding shortfall continues.
«Now we have the facility to provide food to those who are in South Sudan from here (in Sudan), but the level of food is so low that we cannot» provide them what they need, he told AFP in Khartoum.
In recent months Khartoum has opened several «human corridors» to deliver aid directly from Sudan to areas of South Sudan, a move praised by UN and US officials.
Around 403,000 South Sudanese refugees have arrived in Sudan since the war began, the UN says, about 39 percent of them in 2017.
Akodjenou said international donors must acknowledge the «tragic situation», or «mass starvation» among refugees cannot be ruled out.
While a political solution was urgently needed to stop the violence in South Sudan, he said the United Nations also needed to «receive resources to tackle the situation of refugees and IDPs (internally displaced people)».