Tensions escalated in March when an armed group loyal to Machar overran an army base and the government responded with airstrikes.

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.