Fighting in south Sudan's Jonglei state has left some 750 people dead, with growing insecurity in the wider region hampering aid efforts, United Nations officials said on Wednesday. Clashes in Pibor County sparked by cattle raids began earlier this month between the Murle and the Lou branch of the rival Nuer, one of the two main ethnic groups in the south. "We have seen reports of up to 750 dead, that seem to have been caused by cattle rustling," said Geoff Wordley, assistant representative of the UN High Commissioner for Refugees (UNHCR) in south Sudan. "We have seen a number of disturbing and alarming events in terms of security in recent weeks," Wordley told a foreign news agency.