The country's highest court today upheld the death sentence of a white man convicted of killing a black policeman and trying to make the slaying look like the work of black radicals. Henry Burt, 35, faces hanging unless President P.W. Botha commutes the sentence. No execution date was announced. Burt was convicted last year of beating Sgt. Johannes Ndimande into unconsciousness in June 1986, placing a gasoline-soaked tire around his neck, and setting it on fire. No clear motive was established in the trial. That method of killing is called ``necklacing.'' The government says scores of blacks were ``necklaced'' from 1984 through 1987 by black radicals who suspected them of collaborating with white authorities. Burt, a former soldier who worked for the state-run Nuclear Development Corp., pleaded innocent at his trial, though he said he gave the policeman a lift that night. Part of Burt's defense was that he attended a meeting before the killing at which white residents complained about rising black crime. But the Appeal Court rejected that defense in today's ruling.