nairobi south sudan army clash rebel oil-producing region early sunday side say shatter ceasefire hours come effect side accuse start fight around town koch unity state seventeen aid workers flee violence accord humanitarian source immediate detail casualties government rebel group sign ceasefire thursday late attempt end four-year civil war let humanitarian group reach civilians ceasefire formally come force sunday morning fight break soon afterwards accord humanitarian security report see reuters force come heavy fire morning koch county dickson gatluak spokesman government side say force act self defense repulse attack force defeat add statement rebel spokesman lam paul gabriel say army attack first rebel force pursue government soldier towards town world young country plunge war late 2013 president salva kiir sack deputy riek machar dispute erupt fight spread across country largely along ethnic line force loyal kiir dinka machar nuer violence unite nations say amount ethnic cleanse force third population flee early year pocket country plunge briefly famine south sudan army rebel clash hours ceasefire