KAMPALA (Reuters) - South Sudanese rebel groups fought each other this week in a strategic area near the Ugandan border leaving at least three dead, rebels said. The clashes broke out on Tuesday and Wednesday in Kajo Keji between two groups: one loyal to former army general Thomas Cirillo and one loyal to former vice president Riek Machar.      Kajo Keji is a key conduit for rebels moving east and west and an entry point for supplies from Uganda. South Sudan s civil war, triggered by a feud between President Salva Kiir and his former deputy Machar, has plunged parts of the world s youngest nation into famine and forced a third of the population - about four million people - to flee their homes. As the war burns on, battles are being fought on many fronts and armed groups have splintered and begun fighting each other.  The groups that clashed this week both oppose President Salva Kiir s government and had previously been stationed side by side in Kajo Keji.  But SPLA-IO spokesman Lam Paul Gabriel told Reuters that National Salvation Front forces attacked on Tuesday, killing two soldiers and capturing two others. Another SPLA-IO member said they had to withdraw troops from a battle with government troops nearby in order to fight off the attackers. An NSF commander on the ground, John Kenyi Loburon, said SPLA-IO killed one of his men. He said his group captured  a few  rebels. SPLA-IO is former vice president Machar s rebel group, while NSF is loyal to the former general Cirillo.  