U.S.-backed Angolan rebels said they shot down a Soviet-built bomber, destroyed 23 vehicles and killed nearly 50 soldiers, including 12 Cubans, in an attack last week on an armored column. The guerrillas--the National Union for the Total Independence of Angola, led by Jonas Savimbi--said in a statement released in Lisbon that the fighting occurred near the Longa River in southeast Kuando Kubango province. Savimbi announced the start of a new rebel offensive in the 12-year guerrilla war last week. The rebels said they suffered seven dead and 18 wounded in the fighting. There was no confirmation of the fighting from Angola's Marxist government.