With "staggering brutality," South Sudan government soldiers and allied militia targeted civilians by raping them, burning them alive, running them over with armored vehicles and hanging them in trees even as the government pursued a new peace deal to end a civil war, a new Amnesty International report says.

The report released Wednesday, based on interviews with 100 displaced people from Leer and Mayendit counties in Unity State, describes attacks between April and July in what the United Nations has described as an offensive aimed at "clearing opposition-held areas."

The report comes shortly after South Sudan's latest attempt at peace, which was signed last week but has been met with skepticism by the United States and others.

For the first time the United States, Britain and Norway, the troika that helped to achieve South Sudan's independence from Sudan in 2011, decided not to sign the peace deal, saying it remained "concerned about the parties' level of commitment."

Leer and Mayendit have been among the hardest hit regions during South Sudan's five-year civil war, which has killed tens of thousands and made over 2 million people flee the country, creating Africa's largest refugee crisis since the 1994 Rwandan genocide.

Those unable to flee the government offensive in Leer and Mayendit were often killed, with elderly and disabled people burnt alive in their homes, the new report says.

"They raped women, killed old people and took young boys," Nyabieli Gai told The Associated Press during a visit last month to the opposition-held town of Nyal where hundreds of people sought refuge from the offensive. The 50-year-old grandmother fled Mayendit in May when government troops attacked early one morning, raping her daughter-in-law while Gai hid in the bushes, she said.

South Sudan's government, which often dismisses such reports as "rubbish," has said the new peace deal means an end to the war. Any doubt will only fuel the government's resolve "to consolidate peace," President Salva Kiir has said.

Days after the signing, however, fighting broke out between government and opposition forces in Lainya and Kajo Keji counties in Central Equatoria and a Nepalese peacekeeper was shot and wounded in what the U.N. condemned as a "direct attack" on its mission in the town of Yei.

An investigation into the latest fighting is underway by the Ceasefire and Transitional Security Arrangements Monitoring Mechanism, the body charged with monitoring the cease-fire.

At least one conflict expert said the current system for fostering peace in South Sudan is broken.

"Even as South Sudan's leaders talk peace, they continue to wage war. The regional mediators broker cease-fire after cease-fire but show no willingness to enforce them. The parties to the conflict have concluded there is little cost to breaking the agreements and they are right," Alan Boswell, a South Sudan analyst for the Geneva-based Small Arms Survey group that monitors armed violence, told the AP.

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