South Sudan, which split off recently from Sudan, sent the South Sudan Army, known as the SPLA, and the South Sudan Police Service Auxiliary Force to Jonglei in March for Operation Restore Peace.
"Far from bringing security to the region, the SPLA and the police auxiliary forces have committed shocking human rights violations and the authorities are doing very little to stop the abuse," said Audrey Gaughran, Amnesty International's Africa Director.
Survivors describe violence aimed at civilians, including children being assaulted and killed, Amnesty International said.
"I was at home with five other women and our children. They asked us to give guns and we said we didn't have guns and we were beaten with sticks," one woman said. "They took us to pools behind our homes. One soldier stepped on my neck to push my head down and one stepped on my back so I couldn't jump out. My brother's wife was unconscious."