Funding cuts are driving an entire generation of children in Sudan to the brink of irreversible harm as support is scaled back and malnutrition cases persist across the country, the UN children's agency said on Tuesday.

UNHCR and other UN agencies face one of the worst funding crises in decades, compounded by US and other donor states' decisions to slash foreign aid funding.

"Children have limited access to safe water, food, healthcare. Malnutrition is rife, and many good children are reduced to just skin, bones," said Sheldon Yett, UNICEF's Representative in Sudan, speaking via video link from Port Sudan.

Sudan's conflict between the army and rival Rapid Support Forces has displaced millions and split the country into rival zones of control with the RSF still deeply embedded in western Sudan.

Several areas to the south of Sudan's capital Khartoum are at risk of famine, the World Food Programme said in July.

Children were being cut off from life-saving services due to funding cuts, while the scale of need is staggering, UNICEF said.

"With recent funding cuts, many of our partners in Khartoum and elsewhere have been forced to scale back... We are being stretched to the limit across Sudan, with children dying of hunger," Yett said.

"We on the verge of irreversible damage being done to an entire generation of children in Sudan."

Only 23% of the 4.6-billion dollar global humanitarian response plan for Sudan has been funded, according to the United Nations Office for the Coordination of Humanitarian Affairs.

Access to areas in need also continues to be a challenge, with some roads rendered inaccessible due to the rainy season, hampering aid delivery efforts, UNICEF said. Other areas continue to be under siege, such as Al-Fashir.

"It has been one year since famine was confirmed in ZamZam camp and no food has reached this area. Al-Fashir remains under siege. We need that access now," said Jens Laerke of OCHA.