State prosecutors in South Africa have filed an appeal against the conviction and sentence given to athlete Oscar Pistorius for killing his girlfriend.

Last month, Pistorius began serving a five year prison sentence for the culpable homicide of Reeva Steenkamp, although he could be out in 10 months.

The double-amputee Olympic sprinter was cleared of murder.

"The appeal on conviction is based on the question of law," the National Prosecuting Authority spokesman said.

NPA spokesman Nathi Mncube said the state's appeal papers had been filed with the registrar of the North Gauteng High Court.

"This means that the matter is now officially sub judice and consequently the NPA will not be able to discuss the matter publicly until judgment is delivered by the court," he said in a statement.

No further details have been given to explain the grounds for the appeal.

Pistorius is currently serving his sentence in the hospital section of Pretoria's Kgosi Mampuru II jail.

He can apply to serve the rest of his sentence under house arrest after 10 months in prison.

After the verdict, Pistorius's family said that he would not appeal.

Pistorius was charged by the prosecution with the pre-meditated murder of Ms Steenkamp, a model and law graduate.

He was acquitted of this and the lesser murder charge of dolus eventualis.

In South African law, this charge - also known as common-law murder - applies if the accused knew they might kill someone but still went ahead with their course of action.

The athlete was also given a three-year suspended sentence for firing a gun in a restaurant.

The BBC's Pumza Fihlani, who followed the athlete's trial, says the prosecution's grounds for appeal may lie with how the judge interpreted dolus eventualis.

The judge's critics have argued that dolus eventualis includes the possibility of meaning to kill one person and ending up killing another, our correspondent says.

Pistorius says he shot dead Ms Steenkamp in the early hours of Valentine's Day last year by mistake, fearing there was an intruder in the house.