JUBA (Reuters) - At least 58 people were killed in clashes between tribes over cattle in South Sudan, a United Nations spokesman in Juba said Saturday, a further sign of instability weeks after the region gained independence. Ethnic groups in South Sudan have fought each other over cattle - a vital part of the indigenous economy - for centuries. But the numbers of deaths are increasing after decades of civil war left the territory awash with small arms. Analysts say South Sudan, which became independent on July 9, risks becoming a failed state if it cannot control insurgencies and blood feuds that divide its tribes. The UN spokesman said violence between the Murle and Lou Nuer tribes broke out Thursday. "There were around 28 casualties at one location and 30 at another site. We also witnessed a number of burnt Tukuls (huts)," he told Reuters by telephone. "There were several other locations that the UN has not been able to get to. We are relieved that calm appears to be prevailing." From the beginning of the year up to the end of June, 2,368 people have been killed in 330 violent incidents across the South, according to UN data released in July. South Sudan's government has accused the north of arming rival tribes and provoking insurgencies to try to undermine the region and keep control of its oil. Khartoum denies the charge.