The United Nations chief urged regional and international leaders Friday to prevent South Sudan from falling “over the abyss” into another civil war and speak with one voice to support a return to peace.

Both Kiir and Machar are historical leaders of the rebel movement — the Sudan People’s Liberation Movement, or SPLM — that secured South Sudan’s independence from Sudan in 2011. But they are from rival ethnic groups: Kiir is from the Dinka, the largest, and Machar is from the Nuer, the second-largest.

United Nations Secretary-General Antonio Guterres said on Friday that a peace agreement in South Sudan was in a shambles, telling the country's leaders to put down the weapons and put all the people of South Sudan first.