The army was forced out of Bentiu last week
Hundreds
of people were killed because of their ethnic group after South Sudan
rebels seized the oil hub of Bentiu last week, the UN has said.
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.
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In a civil war marked by numerous human rights abuses, the reports from Bentiu are among the most shocking.
The rebels are accused of killing Dinkas (President Kiir's ethnic group), Sudanese (because of the alleged support of Darfuri rebel groups for President Kiir) and Nuers who were not overtly cheering their fellow Nuer rebels.
The victims hid in hospitals and places of worship, but did not find sanctuary there.
Many of the rebels say they took up arms because of the murder of their relatives in Juba at the beginning of this conflict.
Both sides have committed terrible abuses.
However the scale of the killings carried out by rebel troops, including the feared White Army militia, in Bentiu, Bor and Malakal, has turned many people against the rebel leader, Riek Machar.
With the rainy season approaching, and negotiations set to resume in Addis Ababa, there is likely to be more fighting - and very likely more atrocities - in the next few weeks.
Analysis
The rebels are accused of killing Dinkas (President Kiir's ethnic group), Sudanese (because of the alleged support of Darfuri rebel groups for President Kiir) and Nuers who were not overtly cheering their fellow Nuer rebels.
The victims hid in hospitals and places of worship, but did not find sanctuary there.
Many of the rebels say they took up arms because of the murder of their relatives in Juba at the beginning of this conflict.
Both sides have committed terrible abuses.
However the scale of the killings carried out by rebel troops, including the feared White Army militia, in Bentiu, Bor and Malakal, has turned many people against the rebel leader, Riek Machar.
With the rainy season approaching, and negotiations set to resume in Addis Ababa, there is likely to be more fighting - and very likely more atrocities - in the next few weeks.
President Salva Kiir is a member of the country's largest group, the Dinka.
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, 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, tweeted about "shocking scenes of atrocities" in Bantiu, saying "bodies of people executed still lie in the streets".
Many of those killed are understood to be Sudanese traders, especially from Darfur.
Analyst James Copnall says they were targeted because rebel groups in Darfur are alleged to back President Kiir against the rebels.
Video footage from the UN shows bodies lying in the streets of Bantiu
Bentiu, capital of the oil-rich Unity State, has changed hands several times during the conflict.Control of the oilfields is crucial because South Sudan gets about 90% of its revenue from oil.
A ceasefire was signed in January but there has been a recent upsurge in fighting.
Last week, the UN said an attack on one of its bases in the central town of Bor in which at least 58 people were killed could constitute a war crime.
Fighting broke out last year after Mr Kiir accused Mr Machar of plotting to stage a coup.
Mr Machar, who was sacked as vice-president last year, denied the charges but launched a rebellion.
The UN has about 8,500 peacekeepers in South Sudan, which became the world newest state after seceding from Sudan in 2011.
Fighting erupted in the South Sudan
capital, Juba, in mid-December. It followed a political power struggle
between President Salva Kiir and his ex-deputy Riek Machar. The squabble
has taken on an ethnic dimension as politicians' political bases are
often ethnic.
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