The BBC's Alastair Leithead travels to South Sudan's oil hub of Bentiu to investigate reports of rape and murder in one of the worst-affected areas of the nation's brutal conflict.
We met Rebecca in a section of the divided UN compound in Bentiu reserved for ethnic Dinkas and foreigners, and found somewhere quiet and out of sight to speak.
She is 20, visibly pregnant and very nervous describing what happened to her after rebel troops from the rival Nuer community re-took the town last month.
Sexual violence is not something people talk about openly in South Sudan, which became independent in 2011 after seceding from Sudan.
Rebecca says that, as the wife of a Dinka soldier, she was targeted by the rebels.
A translator pieced her story together for me: how she was caught while fleeing to the UN compound, beaten with a rifle butt and told to have sex with two soldiers. She refused and was raped.
"The reason they gave was '[President] Salva Kiir is in charge and is killing our people'," she said.
"They said they wanted to rape me because when Dinka soldiers came here they did the same."
The men told her they wanted to get rid of her Dinka baby, but the attack did not cause a miscarriage.
Her story is similar to many highlighted by a new UN human rights report into atrocities and "crimes against humanity" carried out by both sides in this four-and-a-half month crisis.
Hilde Johnson, head of the UN mission in South Sudan, told the BBC the report showed "how ethnicity has contributed to large-scale violence in the form of mass killings, disappearances, rape and sexual violence of different sorts, abductions, extrajudicial killings".
She added: "South Sudan is now at a tipping point, where we can see an escalation of violence, combined with a humanitarian disaster that can escalate into a major famine at the end of the year.
"We are in a situation where we only have a very short time window for the leaders on both sides to turn the situation around."
In Juba, accounts have been recorded of Nuer women, leaving the UN compound to buy supplies, being grabbed by troops loyal to the government and taken to a hut in view of the base to be raped.
Statements made by at least 10 different women described a pattern of sexual violence based on ethnicity - some of the women were told: "This is what we do - kill your men and rape you."
Thousands of people have now died in the crisis that started on 15 December as a political split between President Kiir and his sacked deputy Riek Machar, and escalated into ethnic violence.
Mr Kiir is a Dinka and Mr Machar a Nuer.
There have been mass killings of people from both ethnic groups, but in Bentiu, foreigners were also targeted.
The government is alleged to be using mercenaries from the Justice and Equality Movement - a notoriously brutal militia that has been blamed for rape and destruction in neighbouring Sudan's Darfur region.
So when rebels loyal to Mr Machar took Bentiu from the government last month, foreigners as well as Dinkas were specifically targeted - anyone suspected of being from Darfur was killed.
The UN human rights report estimates hundreds died in the mosque, church and hospital.
Inside the mosque there is still evidence of what happened two weeks earlier.
It smells of death, there are bullet holes in the walls, belongings and bloodied clothes scattered where they were left when the bodies were removed.
It was many days before they were buried - the dogs and the pigs in the town were feeding on them before mass graves were finally dug.
'Shot eight times'
"First they came in and demanded money and cell phones," said Ismail Yunis.
She is a trader from Sudan who was inside the mosque when the attack happened. She did not want to give her full name.
They were sheltering from the fighting and when rebel troops took control of the town a number of different soldiers came and went, demanding money.
"Then there was a man at the main door with a machine gun, and there were men with Kalashnikov [assault rifles] at every window," she said. "They opened fire and the mosque filled with smoke."
Her sister was next to her - she was shot in the legs and arms.
"She was shot eight times, and when I tried to bring her out they shot her as I held her and she died."
Ismail Yunis saw two men raped in the mosque compound and said more than 100 people were taken outside and summarily killed.
The UN imposed sanctions on Gen Peter Gadet this week in connection with this killing, but it was Maj-Gen James Koang Chol who was in charge of the troops.
He rejected the UN report about what happened under his command when Bentiu was re-taken by the rebels.
"That was rubbish. The UN is biased. Civilians were prevented [from going] to the UN compound by the government - some may have been caught in the middle but that was crossfire, not a massacre."
He also denied accusations that hate speech on the radio was inciting the killing of Dinka men and the rape of Dinka women.
But he admitted they were looking into the reports of the mosque killings.
"We are investigating whether we committed that crime and the perpetrators among us will take the responsibility if there is any," the general said.
The UN is demanding accountability and an independent investigation.
At the UN compound in Bentiu around 30,000 people are now sheltering from the fighting - surviving off limited food and water supplies.
The aid workers are overwhelmed by the demand, the broken supply chain caused by the fighting, and by flooding now it's rainy season.
As the sun was going down, two large rings of children were watching a song and dance being performed to the beat of a plastic-bucket drum.
The UN staff try to bring children from separate Dinka and Nuer sections of the camp to dance together a couple of times a week.
Small steps towards rebuilding ethnic trust in a country that's slipping further towards the abyss.