New York - Hundreds of civilians were massacred in two separate incidents in South Sudan last year in which victims were targeted for their ethnicity, nationality or political views, possibly amounting to war crimes, the United Nations said in a report on Friday.The 33-page report comes after the UN Security Council called for an investigation of April 2014 killings in South Sudan's oil hub, Bentiu. The UN peacekeeping mission in South Sudan, which carried out the probe, also looked at an incident in the same month in the town of Bor."Unmiss Human Rights Division finds that there are reasonable grounds to believe that at least 353 civilians were killed, and at least 250 wounded, in the attacks on Bentiu and Bor," the report said.The Bentiu killings, it said, included at least 19 deaths at a Bentiu hospital and roughly 287 at a mosque in Kalibalek. In Bor, at least 47 civilians were killed at an Unmiss base where they were seeking protection."Perpetrators intentionally targeted civilians, often based on ethnicity, nationality, or perceived support for the opposing party," it said. "In both Bentiu and Bor, attacks took place against protected objects - a hospital, a mosque, and a United Nations base - which may amount to war crimes."The findings could form the basis for war crimes charges. South Sudan is not a party to the International Criminal Court, though the Security Council could theoretically refer the country's 13-month civil war to The Hague-based court.The report urges UN members to support efforts to protect civilians "and assist with accountability efforts".The report quoted one witness to the killings at the Bentiu hospital as saying: "They lined up about 20 Darfurians, who were tied with their clothing ... and told them to run to save their lives. When they ran, [rebel] soldiers shot at them."While rebels appeared responsible for the Bentiu incident, "credible and consistent testimony" suggests the Bor massacre involved government forces, though it was unclear if they were "in their official capacities", the report said.Civil war has killed more than 10 000 people in the world's newest state, reopened deep ethnic divisions, caused more than one million to flee and driven the country of 11 million closer to famine.Violence erupted in December 2013 in South Sudan after months of political tension between President Salva Kiir and his sacked deputy and rival, Riek Machar. The conflict pits Kiir's Dinka ethnic group against Machar's Nuer.The Security Council threatened both sides with sanctions, but is divided over whether to impose an arms embargo on South Sudan, which split from Sudan in 2011.