The army was forced out of Bentiu last week
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.
'Piles of bodies'
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.
He told the BBC's Focus on Africa programme that the scenes
in Bentiu were "perhaps [the] most shocking set of circumstances" he had
ever faced.
He said he saw "piles of [the bodies of] people who had been
slaughtered" last week, adding that they all appeared to be civilians.
Many of those killed were Sudanese traders, especially from Darfur, Mr Lanzer said.
Analyst James Copnall says they could have been targeted
because rebel groups in Darfur are alleged to back President Kiir
against the rebels.
One rebel source said many of those killed in the mosque were actually soldiers who had taken off their uniforms.
Video footage from the UN shows bodies lying in the streets of Bantiu
The situation in South Sudan is "in a downward spiral", Mr Lanzer said, describing the stakes as "very, very high".
There are now more than 22,000 people seeking refuge at the
UN peacekeeping base over the border in Sudan, he said, including
families from the majority community in the state.
"When I asked them why [they were seeking refuge] they said:
'When the violence has such a cycle of revenge you can't tell what will
come next'," Mr Lanzer said.
He added that the UN base was not built for such large
numbers, and that there was currently only one litre of drinking water
for each of the 22,000 civilians in the base, and one latrine for every
350 people.
Upsurge in fighting
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.