NEW : U.N. leader says South Sudan violence shows `` some signs '' of spreading . NEW : South Sudan Red Cross reports 19 killed in Bor , U.N. says . NEW : UN chief Ban Ki-moon calls for an immediate cease-fire and a return to peace talks with the government. NEW : South Sudan government must resume dialogue with opposition , Ban adds .
NEW : U.N. leader says South Sudan violence shows `` some signs '' of spreading . NEW : South Sudan Red Cross reports 19 killed in Bor , U.N. says . NEW : U.N. 's Ban Ki-moon urges president to end the violence that has killed 500 . NEW : South Sudan government must resume dialogue with opposition , Ban adds .