Civil war in South Sudan is spiralling into cycles of revenge far from the control of political leaders, Amnesty International warned Thursday, as pressure builds on commanders to stem the brutal conflict.
"The ethnic dimensions of the conflict are deepening as fighters engage in reprisal attacks, continuously escalating the cycle of violence," Amnesty said in a report, documenting scores of grim testimonies of war crimes in the world's youngest nation.
"The longer ethnic rivalries are allowed to deepen and fester, the more fragmented South Sudan will become, making reconciliation and sustainable peace much more difficult to achieve," it added.
President Salva Kiir and rebel chief Riek Machar are due to meet for talks slated for Friday in Ethiopia, although Machar has already said he will likely not reach Addis Ababa in time.
But the report warns the leaders will struggle to stop the war on the ground even if a deal on paper is reached.
Researchers said they had documented "dozens of mass graves", including five in the war-ravaged town of Bor containing over 500 bodies.
"Horrific atrocities" have been committed by all sides "constituting war crimes and crimes against humanity," the report said.
"Habitual impunity for human rights violations, including international crimes, is a central factor behind repeated cycles of violence," Amnesty said, adding that both sides had "shown total disregard" for the most basic of human rights.
Gunmen on both sides have "deliberately killed civilians (and) executed captured fighters", as well as raping women, burning down homes, destroying medical facilities, and looting food stores and humanitarian aid, it added.
Testimonies in the report describe civilians including children executed by the side of road "like sheep", gang raping of women using sticks, and other victims "grotesquely mutilated" with their lips sliced off.
Fierce fighting continues, and the United Nations has warned of the risk of famine and genocide.
Although starting as a personal rivalry between President Kiir and Machar, the conflict has seen armies divide along ethnic lines and fighting pitting members of Kiir's Dinka tribe against Machar's Nuer.
Those affected feel the "only way to ensure perpetrators are punished and to prevent future abuses is to take the law into their own hands and engage in reprisal attacks," it added.
The United States this week unveiled its first sanctions in response to the "unthinkable violence", targeting one military leader from each side.
The move against presidential force commander Marial Chanuong and rebel general Peter Gadet follows a visit by US Secretary of State John Kerry to Juba last week, where he called on both sides to lay down their arms.
But Amnesty warned more was needed.
"Those up and down the chain of command on both sides of the conflict who are responsible for perpetrating, ordering or acquiescing to such grave abuses... must be held accountable," it said.