Civilians in South Sudan are suffering horrific human rights atrocities, with men being castrated and children being forced to watch their mother's rape and murder, the UN Human Rights Commission has revealed.

UN investigators have collected evidence of human rights violations to hold more than 40 South Sudanese military officials accountable for war crimes and crimes against humanity.

"Conflict-related sexual violence [in South Sudan] is endemic," the Commission said in its report on Friday (23 February).

The report, which chronicles the harrowing testimonies of 230 people, reveals the extent of the cruelty against South Sudanese civilians and how sexual violence is used as a tool of war.

Multiple instances of people having their eyes gouged out and their throats slit were recorded.

Investigators also spoke to men who had been castrated and interviewed an 85-year-old woman who was gang-raped and forced to watch her son and husband being killed.

"The Commission believes the prevalence of sexual violence against men in South Sudan is far more extensive than documented; what we see so far is likely just the tip of the iceberg," said Yasmin Sooka, chair of Human Rights Commission in South Sudan.

The report said many of the cases were "reminiscent of Bosnia," in particular the accounts of civilians who were forced to rape their relatives. One woman told UN investigators that her 12-year-old son was forced to rape his grandmother if he wanted to stay alive. The same woman also had to watch her husband being castrated.

Another pregnant woman said she had seen suspected opposition fighters being tortured and killed. She said she was detained with the decomposing bodies of the victims, including her husband's decapitated body.

UN Commissioner Andrew Clapham said there is a "clear pattern of ethnic persecution" in South Sudan, where tens of thousands of people have been killed and almost four million displaced since the start of the conflict in 2013.

Charu Hogg, director of the All Survivors Project and Associate Fellow on the Asia Program at Chatham House, told IBTimes UK that the documented cases of people being forced to perform sexual acts "falls under the rubric of war crimes.

"Equally, being forced to witness the rape of another person can in itself be considered a form of sexual abuse and may amount to torture or other ill-treatment," she said.

Hogg said that all parties in South Sudan have deployed sexual violence to the conflict. "It is used as a tool to terrorise, intimidate and humiliate," she said. "The breakdown in law and order is a constant challenge as is the lack of availability of basic medical and psychological services for survivors."