Oscar Pistorius' lawyers say prosecutors are persisting with a "failed case" by appealing the double-amputee athlete's acquittal for murder at South Africa's Supreme Court.

Pistorius' lawyers made the argument in papers filed Wednesday to the Supreme Court, meeting Thursday's deadline to respond.

Pistorius was acquitted of murder last year for shooting girlfriend Reeva Steenkamp and was instead found guilty of a lesser charge comparable to manslaughter.

The Supreme Court appeal will be in November, when prosecutors want a panel of judges to overturn the trial verdict and convict the Olympic runner of murder, which calls for a minimum of 15 years in prison.

Pistorius' lawyers argue prosecutors are wrong to challenge the original verdict, but the Supreme Court should order a re-trial if it agrees with the prosecution.