The United States on Monday unveiled some $83 million in new aid for South Sudanese refugees who have fled fighting triggering fears of a looming famine.

The International Committee of the Red Cross warned last week that the situation was critical and the scale of South Sudan's food crisis remained unknown, with aid workers fearing the worst in isolated parts of the conflict-ravaged country.

"Conflict in South Sudan threatens to create a famine in a country where more than two million people are already facing crisis or emergency levels of food insecurity and tens of thousands of children are at risk of malnutrition-related death," the US State Department said in a statement.

"Without progress on political negotiations, the end of the current rainy season is likely to bring a new, intensified chapter of fighting and displacement."

Since fighting erupted in late December between fighters loyal to President Salva Kiir and supporters of his sacked former deputy Riek Machar more than 450,000 people have been forced to flee.

There are now more refugees than when a peace agreement was signed in 2005 ending Sudan's long civil war, the US said.

The new funds would go towards food, health care, water, seeds and farming tools, and brings the total US aid to South Sudan this year to $720 million.

Washington renewed calls on rival factions to end their fighting and allow immediate access for humanitarian aid workers.

"Gains made through international assistance can only be sustained if leaders prioritize peace and invest in services for their own people," the State Department said.