President Jaafar Numeiri of Sudan declared a cease-fire in the two-year bush war in the south and repeated an offer of amnesty to the rebels. In a speech marking the 13th anniversary of the Addis Ababa accords that ended 17 years of civil war, Numeiri said the armed forces' role "will be limited to defense . . . ." Southern rebels have rejected previous such calls, saying they seek Numeiri's overthrow.