Pistorius, an Olympic and Paralympic track star, maintained throughout his seven-month trial that he shot Steenkamp on Feb. 14, 2013, by mistake, thinking she was an intruder hiding behind the bathroom door in his house in Pretoria.
The prosecution failed to convince Pretoria High Court Judge Thokozile Masipa of Pistorius' intent to kill when he fired at his girlfriend, leading to his conviction in October for negligent killing - culpable homicide - and a five-year jail term.
"Based on all the evidence, perhaps the element of mercy was over-exaggerated," state prosecutor Gerrie Nel argued before Masipa
Masipa's decision to rule out murder was criticised by several legal experts and the Women's League of the ruling African National Congress (ANC) as an erroneous interpretation of the law.
Pistorius, who was not in court on Tuesday, and his legal team are opposing the appeal.
In South African law, the judge who handed down a verdict decides on whether the judgement can be appealed.
(Reporting by Dinky Mkhize, Writing by Peroshni Govender,; Editing by Angus MacSwan)
