Hundreds of people were killed because of their ethnicity after South Sudan rebels seized the oil hub of Bentiu last week, the UN has said.
They were targeted at a mosque, a church and a hospital, the UN Mission in South Sudan said in a statement.
It added that hate speech was broadcast on local radio stations, saying certain groups should leave the town and urging men to rape women.
The Nuer community are seen as supporters of rebel leader Riek Machar.
President Salva Kiir is a member of the country’s largest group, the Dinka.
Although both men have prominent supporters from various communities, there have been numerous reports of rebels killing ethnic Dinkas and the army targeting Nuers since the conflict broke out in December 2013.
Since then, more than a million people have fled their homes in what was already among the world’s poorest nations.
South Sudan analyst James Copnall says that in a civil war marked by numerous human rights abuses, the reports from Bentiu are among the most shocking.
Non-Nuer South Sudanese and foreign nationals were singled out and killed, the UN Mission in South Sudan (Unmiss) said.
Some 200 civilians were reportedly killed at the Kali-Ballee mosque where they had sought shelter.
At the hospital, Nuer men, women and children, who hid rather than cheer the rebel forces as they entered the town, were also killed, it said.
The UN’s top humanitarian official in South Sudan, Toby Lanzer, was in Bentiu on Sunday and Monday.