An outbreak of cholera is putting thousands of lives at risk in parts of South Sudan, worsening the country's humanitarian crisis amid ongoing violence, according to the aid group Save the Children.

The disease has infected more than 2,600 people and killed at least 60 since the first cases were reported in the capital, Juba, in May.

Cholera outbreaks or alerts have been reported in nine of South Sudan's 10 states.

The situation is likely to get worse with heavy rains expected in coming weeks and months, the aid group said, calling for greater supplies of medicines.

"Stagnant floodwater provides the perfect conditions for the rapid spread of cholera and roads are turning to mud, hindering efforts to get support and life-saving drugs to those that desperately need them," Pete Walsh, Save the Children's South Sudan director, said on Friday.

He noted that the spread is "extremely concerning, especially coming on top of a growing hunger crisis and as hundreds of thousands of people are struggling to survive in overcrowded, unsanitary camps".

Children are more vulnerable to cholera, which causes severe dehydration that can be fatal.

The disease is treatable if detected in time.

The world's newest country descended into chaos last December after President Salva Kiir accused his former deputy, Riek Machar, of trying to launch a coup.

A ceasefire signed earlier this year has repeatedly been violated by both sides, according to observers.

More than 400,000 South Sudanese have fled their country, seeking refuge in Ethiopia, Uganda, Kenya and Sudan. More than one million South Sudanese remain internally displaced.