War-torn South Sudan is in a "spiralling humanitarian crisis", the UN warned Monday, as top aid chiefs visited the young nation, where more than a million people have fled months of conflict.

"People are in acute need," UN World Food Programme (WFP) boss Ertharin Cousin said as she arrived for a two-day assessment.

"Large-scale population displacement and disruption of markets and trade routes are creating a food security crisis," she said in a statement. 

More than 800,000 people are displaced inside South Sudan, while almost 255,000 have fled to the neighbouring countries of Ethiopia, Kenya, Uganda and Sudan, the UN says.

Many had only returned to South Sudan after it won independence from Sudan in July 2011, following the end of the 1983-2005 civil war.

"It is heartbreaking to see that some of the very people who had fled the war two decades ago, people we helped to return to South Sudan after independence, are having to flee for their lives again, many back to the very same places where they lived in exile," UN refugee agency chief (UNHCR) Antonio Guterres said.

Violence erupted in South Sudan on December 15 between forces loyal to President Salva Kiir and fighters loosely allied to former vice president Riek Machar.

A ceasefire between government and rebels signed in January is in tatters with fighting ongoing.

Cousin said aid workers needed both "safe access" to support those affected by the fighting and more funds to provide "life saving supplies" including food, shelter and medicine.

"Several countries have contributed generously, but at current levels we are only able to cover a fraction of the needs," she added.

The UN appeal for funds for South Sudan is only a quarter funded, with almost five million people in need of aid.

READ: UN body needs $59m for South Sudanese refugees

WFP says it needs $224 million in South Sudan over the next six months, while UNHCR has called for more than $370 million to help refugees in neighbouring countries.