Pro-government forces in South Sudan are being allowed to rape women and children as a form of payment, according to a UN report.

In one incident soldiers argued over whether to rape a six-year-old girl and then shot her.

The UN human rights office said its report contained "harrowing accounts" of civilians suspected of supporting the opposition - including children and the disabled - being burned alive, suffocated in shipping containers, hanged from trees and cut to pieces.

It said the crimes uncovered showed the war-torn country was facing "one of the most horrendous human rights situations in the world".

A UN assessment team, which compiled Friday's report, was deployed to the country in 2015 and recorded more than 1,300 reports of rapes in South Sudan's Unity State alone.

UN human rights chief Zeid Ra'ad Al Hussein said the number "must only be a snapshot of the real total".

"The assessment team received information that the armed militias... who carry out attacks together with the SPLA (South Sudanese army) commit violations under an agreement of 'do what you can and take what you can,'" the report said.

"Most of the youth therefore also raided cattle, stole personal property, raped and abducted women and girls as a form of payment."

The report also cited cases of parents being forced to watch their children being raped, and warned even women and youngsters inside UN protected camps were at risk.

Last month, at least 18 people were killed in attacks at a UN base sheltering civilians in the northeastern town of Malakal.

The violence forced around 600 people - mostly women and children - to gather inside a Medecins Sans Frontieres (MSF) hospital.

UN Secretary-General Ban Ki-moon has condemned the fighting and expressed concerns about the rise of ethnic violence in the conflict, which began in December 2013.