Twelve black nationalist guerrillas were killed and three members of South Africa's security forces wounded in a clash in the Caprivi Strip, near the Zambian-Angola border, the government said today.

Brig. D.J. Earp, director of Air Force operations, said guerrillas of the Southwest African People's Organization attacked a temporary patrol base about 5 miles from the border on Tuesday. The Caprivi Strip, a part of the Southwest Africa, is a long projection surrounded by Angola to the north, Zambia to north and east and Botswana to the south.

In other southern Africa developments: In Cape Town, riot police reported killing one black youth and wounding another to prevent a crowd from attacking a house in Guguletu black township on Thursday.

The leader of Transkei's opposition Democratic Party, Hector Ncokazi, and two other party officials were freed after being detained since July for allegedly having advocated reincorporation of the country into South Africa. Another party official was rearrested several days ago.

Rhodesian Prime Minister Ian Smith came under strong editorial criticism in Salisbury for allegedly not going far enough toward his stated goal of abolishing racial discrimination. The independent Herald said Smith's recent announcements "could all add up to another lost opportunity."