Government soldiers follow orders to raise their guns during a military parade in Juba, South Sudan in April. Two million people have been displaced by violence.Picture: AP

Geneva - South Sudan's civilians had their eyes gouged out, were castrated and were forced to rape each other by the country's warring sides, according to UN rights investigators who have collected evidence against more than 40 senior military figures.

The UN Commission on Human Rights in South Sudan on Friday presented information pointing to crimes against humanity and war crimes committed during the past two years amid the world's fastest-growing refugee crisis.

Children have been recruited by all sides of the conflict and have been forced to kill civilians, the commission found.

"Conflict-related sexual violence is endemic," its report said.

Children are thought to make up a quarter of sexual violence victims.

Some civilians were forced to rape close family members. One of the 230 witnesses interviewed by the commission said her 12-year-old son was coerced to have sex with his grandmother in order to stay alive.

A split between President Salva Kiir and his former deputy, Riek Machar, escalated into war in 2013. Tens of thousands have been killed and more than 4 million displaced.

Commission chairwoman Yasmin Sooka said the evidence against senior officials should be used by the so-called Hybrid Court, which South Sudan agreed to set up with the African Union in 2015, but which has yet to materialize.

"Ultimately, this is the only way to stop the rampant devastation of millions of human lives by South Sudan's leaders," said Sooka.

The report was prepared for the UN Human Rights Council, which starts its month-long spring session on Monday.