The investigators, from the UN Commission on Human Rights in South Sudan, say their findings are based on interviews with hundreds of witnesses, satellite imagery and almost 60,000 documents dating back to the outbreak of the war.
United Nations 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.
Across South Sudan, five years of ongoing conflict has uprooted more than four million people but peace efforts are under way both internationally and at grassroots level to end violence.
But the court has still not been set up because South Sudan's parliament has not yet approved it.
"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 Sooka.
Since then, tens of thousands of people have died and more than 4.5 million have been forced to flee their homes.
"The UN has very firm view that we do not support the death penalty", said the head of the UN Mission in South Sudan (UNMISS) David Shearer on Thursday in reference to the trials. "The suffering and cruelty was worse than anyone could have imagined", said Andrew Clapham, a commission member and worldwide law professor. Among accounts it accumulated was one from a South Sudanese man who said he had been hiding in the bush and returned home to find that government soldiers had gouged out the eyes of his wife with spears when she tried to stop them from raping their 17-year-old daughter. An attempt at a ceasefire in late December was violated within hours.
But no high-ranking officials have been held to account, despite African Union (AU) promises to establish a special court to try alleged crimes. The NSS' broad powers of surveillance, arrest and detention, including the embedding of its officers in some media offices, meant that people were deterred from participating and discussing the state of the country or human rights, the report stated.
"These acts constitute war crimes and crimes against humanity", the commission's report said.
Endley formerly served as an adviser to rebel leader Dr Riek Machar, the leader of the Sudan People's Liberation Army in Opposition (SPLA-IO) who is now in exile in South Africa.
"The recommendations of these human rights reports have not been implemented in South Sudan, making them useless", said Jacob Chol, senior political analyst and professor at the University of Juba.
Elections are due in South Sudan this year, according to the 2015 peace pact.
The talks reached a stalemate a week ago but are expected to pick up again next month.