Animosity between South Sudan's Dinka ethnic group and the people of its southwestern Equatoria region surged earlier this month when buses on roads south of the capital Juba were attacked as they passed through Equatoria region.
The government said 21 civilians were killed in one ambush on October 8, but rumours that as many as 200 Dinka had been killed circulated on social media, followed by calls for revenge.
Letters threatening violence and "elimination" of Equatorians in the north-western region of Aweil and an attack on an Equatorian humanitarian worker in Aweil town prompted the evacuation of nearly 100 humanitarian staff from the area, the statement from Zeid Ra'ad al Hussein said.