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Ben Curtis, File, Associated Press
In this Friday Dec. 27 , 2013 file photo, displaced people bathe and wash clothes in a stream inside a United Nations compound which has become home to thousands of people displaced by the recent fighting, in Juba, South Sudan. One year after mass violence broke out in South Sudan, battles between government forces and rebel fighters continue, and aid officials say the international community must help residents stave off mass hunger.

NAIROBI, Kenya — Tens of thousands of people have died in South Sudan during one year of warfare and the country's leaders are putting their "personal ambitions" ahead of the nation's future, the U.N. secretary-general said Monday.

A year ago Monday fighting broke out in South Sudan's capital, Juba, and spiraled across the country. The U.N. says more than 1.9 million people have been displaced by the warfare, battles that often pit fighters loyal to President Salva Kiir, an ethnic Dinka, against those who support former Vice President Riek Machar, an ethnic Nuer.

U.N. Secretary-General Ban Ki-moon called on South Sudan's leaders to agree to an inclusive power-sharing arrangement that would address the root causes of the conflict and ensure accountability for any crimes committed on the battlefield.

There is no official death toll for the conflict, but Ban said "tens of thousands" of South Sudanese have died. The fighting has been marked by vicious atrocities, largely ethnic in nature.

The two sides have signed several peace deals brokered by neighboring governments, but none has succeeded in stopping the fighting in the oil-rich country.

South Sudan's civilians face a "dreadful" situation and have been victims of targeted killings and looting, said the U.N. High Commissioner for Human Rights, Zeid Ra'ad Al Hussein.

"The people of South Sudan are living in a tinderbox, with emotions high, an abundant flow of weapons and with both sides recruiting fighters, often forcefully and including children," Al Hussein said.

Government troops and armed youths have been battling in Upper Nile state in recent days, a sign that widespread violence could return now that the six-month rainy season has ended.