South Sudanese refugees cross a wooden bridge from South Sudan to Uganda at the Busia crossing, near Kuluba, in northern Uganda. File picture: Ben Curtis/AP

Juba - At least thirty people were killed in tribal clashes in South Sudan's northern Tonj state on Friday, officials said.

The fighting between two sub-clans of the dinka tribe came days after President Salva Kiir removed the state's governor and replaced him with another.

"We have confirmed thirty bodies, including women and elderly people, from both sides," the new governor, Simon Madut Aleu, told dpa.

"Despite the government's initiative to restore peace among locals through disarmament and dialogue, tribal clashes and cattle raiding remain the major challenges in the state," he said.

Cattle raiding has occurred for centuries in the East African nation, and children are sometimes abducted during raids to be used as domestic slaves. The raids often carry an ethnic component.

South Sudan is in its fifth year of civil war despite the signing of a ceasefire agreement in December. The conflict, sparked by a 2013 split between Kiir and Machar, his former deputy, has led to tens of thousands of deaths and the displacement of 2.5 million people.

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.

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.