Rebel gunmen in South Sudan massacred hundreds of civilians in ethnic killings when they captured the oil town of Bentiu last week, the UN says.
It is one of the worst reported atrocities in the war-torn nation.
In the main mosque alone, more than 200 civilians were reportedly killed and more than 400 wounded, the UN mission in the country said on Monday.
There were also massacres at a church, hospital and an abandoned UN World Food Program (WFP) compound.
Fighters took to the radio calling for rival groups to be forced from the town and for men to rape women from the opposition ethnic group.
South Sudan's army has been fighting rebels loyal to sacked vice president Riek Machar after the insurgents launched a renewed offensive targeting key oil fields.
The conflict has also taken on an ethnic dimension, pitting President Salva Kiir's Dinka tribe against militia forces from Machar's Nuer people.
UN human rights investigators said that after rebels wrested Bentiu from government forces in heavy battles last Tuesday, the gunmen spent two days hunting down those they believed opposed them.
Both South Sudanese and Sudanese, some from the war-torn Darfur region, were killed, the UN peacekeeping mission in South Sudan (UNMISS) said in a statement.
"They (the rebels) searched a number of places where hundreds of South Sudanese and foreign civilians had taken refuge and killed hundreds of the civilians after determining their ethnicity or nationality," the UN said.
Some rebels took to the local radio to "broadcast hate messages declaring that certain ethnic groups should not stay in Bentiu and even calling on men from one community to commit vengeful sexual violence against women from another community," the statement added.
At the Kali-Ballee mosque, where hundreds had taken shelter, the rebels "separated individuals of certain nationalities and ethnic groups and escorted them to safety, while the others were killed", the report raid.
At the hospital, "several Nuer men, women and children were killed for hiding and declining to join other Nuers who had gone out to cheer" the rebels as they entered the town, the UN said.
Similar killings were reported at the Catholic church and WFP compound.
Peacekeepers have already said they had seen dozens of corpses in military uniform on the streets of the northern town, state capital of Unity state.
The capture of Bentiu came two days before gunmen stormed a UN compound in which at least 58 people were killed, with peacekeepers fighting back to protect more than 5000 civilians sheltering there who the attackers planned to kill.
