On the occasion of the third anniversary of South Sudan's independence, Ban recalled in a statement the hopes and expectations of the South Sudanese people when their country was established on July 9, 2011, Xinhua reported.
"Those hopes were dashed by the conflict that broke out in December 2013. Thousands of South Sudanese have been killed, and atrocities have been committed against civilians," the statement said.
The world's youngest country has been thrown into turmoil since Dec 15, when political infighting between President Salva Kiir and former vice president Riek Machar turned into a full-fledged conflict.
According to the UN, escalating violence has since then uprooted some 1.5 million people and placed more than 7 million at risk of hunger and disease.
In the statement, the UN chief said the South Sudanese people, who are bearing the brunt of the failure to stop the fighting, are living in squalor. "Their livelihoods have been lost and they are plagued by hunger, disease and insecurity."
"The Secretary-General reminds the leaders of South Sudan that this is a man-made crisis. It is their responsibility and within their power to stop it," the statement said, urging the leaders to "live up to the expectations of their people, lay down their arms and return immediately to the negotiation table."