Ban Ki-moon called on the country'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 UN Security Council blamed South Sudan's "man-made political, security and humanitarian catastrophe" on its feuding leaders on Monday, as the world body threatened targeted sanctions against those impeding the peace process.
UN High Commissioner for Human Rights, Zeid Ra'ad Al Hussein, said that civilians faced a "dreadful" situation and were victims of targeted killings and looting.
"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," Hussein said.
War broke out in the world's newest nation a year ago on Monday, when President Salva Kiir accused his deputy Riek Machar of trying to organise a coup.
More than 1.9 million people were displaced by the fighting, the UN says, with sectarian battles pitting militias loyal to Kiir, an ethnic Dinka, against those who support Machar, an ethnic Nuer.
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, Al Jazeera reported.
In recent days, government troops and armed youths have been battling in Upper Nile state, a sign that widespread violence could return now that the six-month rainy season has ended.
Rights groups say the country is locked in conflict, with the bloodshed that erupted in Juba a year ago having set off a cycle of retaliatory massacres across large swathes of the country.