KAMPALA, Uganda — South Sudan’s leaders have amassed wealth abroad amid a conflict in which tens of thousands have been killed, a U.S.-based watchdog group said Monday, charging that the civil war is being fueled by competition among rivals over national resources such as oil. South Sudan President Salva Kiir, former deputy Riek Machar and those close to both men have looted the country in accumulating wealth that includes mansions, luxury cars and stakes in a number of businesses abroad, according to the report by The Sentry. The report says it has obtained images of officials’ family members jet-setting and partying in five-star hotels, as well as documentation of their properties abroad.