Juba - Rebels in South Sudan have slaughtered at least 353 civilians in April last year, including people sheltering in a mosque, hospital and a United Nations base, UN rights investigators said on Friday.
The report is the first detailed account of two incidents that have highlighted a pattern of gross abuses and atrocities committed during the year-old civil war in the world's youngest nation.
The UN also noted that nearly nine months after the events, "no perpetrator has been held accountable" for the killings.
In the 15 April attack on the northern oil-town of Bentiu, fighters backing South Sudan rebel leader Riek Machar murdered at least 287 civilians sheltering in a mosque, many of them traders and their families from neighbouring Sudan's Darfur region.
Later that day, 19 civilians were killed in the town's hospital, UN investigators said.
Also Read: China sends 700 peacekeepers to South Sudan
Fighters also took to the radio urging rival groups to be forced from the town and for men to rape women from the rival tribe.
"Victims were deliberately targeted on the basis of their ethnicity, nationality or perceived support for one of the parties to the conflict," the UN said in a 33-page report.
Two days later on 17 April, in the eastern town of Bor, a gang of heavily-armed men marched on the UN base, where hundreds of civilians had fled to for protection.
The "mob forcibly entered the protection site and went on a rampage of killing, looting and abductions", the report by the UN peacekeeping mission in South Sudan (Unmiss) said, adding the assault left at least 47 dead and was likely to have been "planned in advance".
The attack on Bentiu and the UN base in Bor are two of the most high profile massacres in a long list of atrocities carried out in the more than a year-long civil war.
At least 353 civilians were killed and another 250 were wounded in the two attacks, the UN said.
Also Read: Tens of thousands dead in South Sudan conflict: UN
No overall death toll for the war has been kept, either by the government, rebels or the UN, although the International Crisis Group says it estimates that at least 50 000 people have been killed.
Some diplomats suggest it could be double that figure, while hunger and disease have killed thousands more.
Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when President Kiir accused his sacked deputy Riek Machar of attempting a coup.
The fighting in the capital Juba set off a cycle of retaliatory massacres across the country, pushing it to the brink of famine. Both government forces loyal to Kiir and rebels loyal to Machar continue to fight, despite numerous ceasefire deals.