Why did the united nations impose sanctions on rhodesia after independence?

When Rhodesia under the leadership of Prime Minister Ian Smith declared independence on November 11, 1965, the United Nations adopted Resolution 216 condemning the country’s move to declare independence as an act by a racist minority.  Britain also refused to recognize the new government and imposed economic sanctions on Rhodesia such as ending the link between the sterling and Rhodesian currency, seizing assets and banning the import of Rhodesian tobacco.  Rhodesia later reconstituted into Zimbabwe Rhodesia in June 1, 1979.