A Christian network of aid charities, Caritas, said unending violence in the country means millions of people need help just to survive from day to day.
A statement issued by the local president of Caritas, Bishop Erkolano Lodu Tombe of Yei, said South Sudan is becoming a "forgotten crisis".
"People are living in constant fear and insecurity, suffering mentally, physically and are starving," Caritas South Sudan has warned. "In the coming months heavy rains will add to the already untold suffering for the people."
The world's youngest country, which achieved independence in July 2011, has been divided since December 2013 by fighting between forces loyal to President Salva Kiir and followers of the former Vice President, Dr Riek Machar. The conflict has escalated as other local militias join the fighting.
Caritas has called for an end to tribal manipulation, which is fuelling violence between communities.
The UN estimates that between 50,000 and 300,000 people have been killed and an untold number injured in the conflict. An estimated 4.5 million people have been displaced, either living in makeshift camps, or fleeing to neighbouring Uganda, Ethiopia, Sudan, and Kenya.
