Prosecutors have begun an appeal against the verdict in the Oscar Pistorius case, arguing he should have been convicted of murder.

The athlete was convicted of culpable homicide - equivalent to manslaughter - for shooting dead girlfriend Reeva Steenkamp in February 2013.

He is due to be released on parole on Friday after 10 months in jail and is expected to wear a tracking tag.

Pistorius was sentenced to five years but will be freed in accordance with guidelines for non-dangerous inmates.

He was not found guilty of murder because the judge said the prosecution had not proved he intended to kill when he opened fire on his toilet door.

Prosecutors disagree and say the verdict should be changed to murder because he did have that intention.

Pistorius said he believed there was an intruder in his property but during the trial it was claimed he deliberately killed Ms Steenkamp after an argument.

"We filed the appeal today," said National Prosecuting Authority spokesman Luvuyo Mfaku.

Pistorius' lawyers have a month to respond and the Supreme Court is expected to hear the appeal in November.

If the appeals court finds him guilty of murder, the athlete - known as the Blade Runner for the prosthetic legs he wears on the track - could face at least 15 years in jail.

A statement from Ms Steenkamp's parents, reacting to the appeal, said: "We are still struggling with coming to terms with losing our precious daughter Reeva and her loss is felt even more this week as we would have celebrated her 32nd birthday on 19 August."

"As a family we want to continue to honour Reeva's memory and will commemorate her birthday by focusing on the Reeva Rebecca Steenkamp Foundation, celebrating her life, what she stood for and the joy she brought into so many lives."

In a previous letter, Ms Steenkamp's family told the parole board "10 months for taking a life is simply not enough".