South African authorities are challenging Olympian Oscar Pistorius appeal against his conviction for the murder of girlfriend Reeva Steenkamp, the national prosecuting authority said on Monday.

The Supreme Court of Appeal upgraded the 29-year-old's conviction to murder in December after the state prosecutors appealed the athletes April conviction for culpable homicide in the Pretoria High Court.

In the affidavit, prosecutors argue that the Supreme Court of Appeal had correctly found Pistorius guilty.

Pistorius shot and killed Steenkamp on Valentine's Day in 2013, and has spent the last few months under house arrest at his uncle's home in Pretoria while awaiting his appeal.

Pistorius's lawyer Barry Roux has said the appeal would cite grounds including the appeal court reconsidering a trial court finding that Pistorius felt his and Steenkamp's life were in danger when he fired the shots.

Pistorius, if his appeal fails, faces a possible minimum 15-year jail sentence for the murder of his girlfriend.

Image: Oscar Pistorius stands in the dock at the North Gauteng High Court in Pretoria for a bail hearing on December 8, 2015 

Photograph: Herman Verwey/Reuters