South Africa's Paralympic athlete Oscar Pistorius appeared in court Monday for a sentencing hearing set to send him back to jail for murdering his girlfriend Reeva Steenkamp three years ago.

The double-amputee killed Steenkamp, a model and law graduate, in the early hours of Valentine's Day in 2013, saying he mistook her for an intruder when he fired four times through the door of his bedroom toilet.

Wearing a dark suit, white shirt and black tie, Pistorius attended the hearing along with members of his family.

A lawyer close to the case said the sentencing could take place on Friday.

Steenkamp's relatives, including mother and father, were also present as proceedings opened in the High Court in Pretoria.

In March, Pistorius's lawyers failed in their legal bid to reverse a Supreme Court of Appeal judgement that upgraded his original conviction from culpable homicide -- the equivalent of manslaughter -- to murder.

The original trial judge Thokozile Masipa is expected to pass a new sentence after hearing arguments from both prosecution and defence.

The prosecution will be pushing for a tougher penalty.

The 29-year-old "Blade Runner" faces a minimum 15-year jail term for murder, but his sentence could be reduced due to time already spent in prison and mitigating factors, including his disability.

Legal experts say Pistorius could take the stand to express remorse.

"Contriteness is a very important sentencing factor. If someone apologises and shows remorse, that is a strong mitigation factor," said criminal law expert Martin Hood.

Steenkamp's father Barry may also take the witness stand to ask the court to impose a lengthy punishment.

If he does, it will be the first time one of Steenkamp's parents has testified in the case.

Pistorius was released from jail last October to live under house arrest at his uncle's mansion in Pretoria after serving one year of his five-year sentence for culpable homicide.

- Minimum 15-year jail -

If he is sentenced to more than five years this time around, he will have to serve at least two-thirds of the term before he qualifies for parole.

Analysts estimate that he could be sentenced to between eight and 12 years, but the prosecution is pushing for at least 15.

"When it is not premeditated and when a person is a first offender, (murder) carries a minimum of 15 years... and we have the responsibility to ensure that the provisions of the law are applied," said the National Prosecution Authority's spokesman Luvuyo Mfaku.

Since being convicted of murder, Pistorius has been on bail and allowed to leave the house at set times, but not travel further than 20 kilometres (12 miles) without permission.

The case -- held in the same courtroom as the original trial -- was scheduled to last one week, though the sentence could be handed down earlier.

A lawyer close to the case told AFP on condition of anonymity that it may take three days for both sides to present evidence and "the judge may come back to deliver sentence on Friday."

Pistorius has always denied killing Steenkamp in a rage and, during his dramatic seven-month trial in 2014, sobbed in the dock as details of his lover's death were examined in excruciating detail.

Pistorius has shunned the media during years of intense coverage since Steenkamp's killing, but his family have revealed that he has given his first interview, due to air on British broadcaster ITV later this month.

The year before he killed Steenkamp, Pistorius became the first double-amputee to race at Olympic level when he appeared at the London 2012 games.

He has since lost his glittering sports career, lucrative contracts and status as a global role model for the disabled.