Human Rights Watch accused SPLA of killing, torturing, raping and detaining civilians in and around the western town of Wau.
The body in its statement on Tuesday also accused the army of looting and burning down homes during government respond to SPLM/A-IO operations that intensified after the peace deal.
It called on the transitional government of national unity to take immediate steps toward accountability for crimes by all warring parties since the start of the conflict in December 2013.
Africa Director at Human Rights Watch, Daniel Bekele urged the government to immediately stop the abuse, free all detained civilians and create war crimes court that can prosecute perpetrator.
He said the agency found during its research mission to Wau in April 2016 that the abuses have forced tens of thousands of people to flee their villages.
The body said it documented the army’s similar abusive counterinsurgency tactics in Western Equatoria in February 2016.
CRN could not reach SPLA for comments.