"The wanton destruction and violence against civilians in this conflict is shocking," Daniel Bekele, Africa director at Human Rights Watch, said in a statement from neighboring Kenya.
Conflict erupted in December when South Sudanese President Salva Kiir, from the Dinka ethnic community, accused former Vice President Riek Machar, from the rival Nuer community, of trying to overthrow the government. Machar was sidelined when Kiir reshuffled his Cabinet in July.
Bekele's team visited Malakal, the capital of Upper Nile state, and Bentiu, the capital of Unity state, in mid February to get a first-hand account of the violence. There, they found forces from both sides of the conflict had targeted civilians and also summarily executed some based on their ethnic background.
Bekele said the two state capitals are "extensively destroyed" and threats of more ethnic violence had forced much of the local population to abandon their homes.
"Military commanders from both sides have an obligation to immediately and unequivocally order their forces to stop attacking civilians and civilian property, and the commanders need to hold abusive soldiers to account," he said.
South Sudan's independence in 2012 came out of a 2005 peace agreement that ended Sudan's civil war.