Human Rights Watch is calling for the African Union to establish a court to try the most serious cases of abuses in South Sudan where it says both the army and rebels have committed serious violations.
HRW said Monday there is evidence of killings, rapes and forced disappearances by government forces in South Sudan's Western Equatoria state.
Last month, U.N. Assistant Secretary-General Ivan Simonovic highlighted Western Equatoria as one of the new areas where violence was taking place and that he had received reports of killings, sexual and gender-based violence and the destruction of property there.
HRW said its researchers visited the area in February, with witnesses describing soldiers attacking homes, looting belongings and displacing thousands of civilians.
Violence in South Sudan has persisted despite a peace accord signed in August between the government and armed opposition.
"As South Sudan's fighting has shifted west, so too have the atrocities by government forces and rebel groups," said HRW Africa Director Daniel Bekele. "South Sudan's leaders should put a stop to all abuses."
HRW and other groups have also called for an arms embargo on South Sudan.
During the past two years, tens of thousands of people have died and more than 2 million have been displaced because of the violence.