"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 rights office said in a new report.
"Most of the youth therefore also raided cattle, stole personal property, raped and abducted women and girls as a form of payment," the report added.
In a report, the UN human rights office painted a harrowing picture of civilians suspected of supporting the opposition, including children, being burned alive, suffocated in shipping containers, hanged from trees and cut to pieces.
UN human rights chief Zeid Ra'ad Al Hussein meanwhile warned that brutal rapes had been used systematically as "an instrument of terror and weapon of war."
"This is one of the most horrendous human rights situations in the world," he said in a statement.
After gaining independence from Sudan in 2011, South Sudan erupted into civil war in December 2013, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.