South Sudan rebels killed, raped and kidnapped civilians during an attack in October, leaving at least 11 dead, the United Nations said in a report.
Fighters backing former vice president Riek Machar attacked the oil-rich town of Bentiu near the border with Sudan on October 29 and "killed at least 11 civilians and committed other serious human rights abuses," said the report, received by AFP on Saturday.
The UN Mission in South Sudan also said it had "received testimony from multiple sources alleging that opposition forces abducted and raped female residents of Bentiu after government troops withdrew from some parts of the city during the afternoon of 29 October 2014."
"According to several witnesses, two women and a six-month-old baby were killed in their homes by opposition forces," said the report, compiled after an investigation by the UN mission's human rights division.
 "According to its preliminary findings (opposition) forces committed gross human rights abuses and serious violations of international humanitarian law which may amount to war crimes."
Bentiu has been hotly contested between the two sides and its control has changed hands several times.
After briefly wresting control of the city in April, Machar's forces killed hundreds of residents, according to the United Nations. People were even slaughtered in mosques and hospitals, it said.
The town is now nearly deserted, and some 44,000 people are sheltering at the UN base in the South Sudan capital Juba.
Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when President Salva Kiir accused his sacked deputy Riek Machar of attempting a coup.
The fighting in Juba set off a cycle of retaliatory massacres across large swathes of the country, pushing it to the brink of famine.
The International Crisis Group estimates that at least 50,000 people have been killed, while some diplomats suggest the death toll could be double that figure.
Peace talks aimed at ending South Sudan's civil war resumed in Ethiopia's capital Addis Ababa on Thursday, with mediators making a fresh appeal for an end to a "year of horror and tragedy".