Olympic and Paralympic track star Oscar Pistorius looks on ahead of his sentencing hearing at the North Gauteng High Court in Pretoria, in this October 16, 2014 file photo. [Photo/Agencies]

BLOEMFONTEIN - Paralympian Oscar Pistorius' conviction for killing his girlfriend Reeva Steenkamp has been scaled up to murder from culpable homicide by South Africa's top appeals court.

"Blade Runner" Pistorius could now be sent back to jail for at least 15 years for shooting Steenkamp dead on Valentine's Day 2013.

The athlete is expected to be sentenced for the new murder conviction by a lower court at a date still to be determined.

Last year a judge gave Pistorius a five-year jail sentence for "culpable homicide" of Steenkamp, but prosecutors argued that he should be convicted of murder for firing four shots through a locked toilet door in a case that attracted interest around the world and continues to fascinate and divide South Africa.

Pistorius left jail on parole in October and is meant to serve the rest of his sentence under house arrest.

"This is a human tragedy of Shakespearean proportions," Judge Eric Leach said as he started reading the ruling.

State prosecutors who lodged the appeal say Pistorius intended to kill Steenkamp and that she fled to a toilet during a row. Pistorius denies deliberately killing Steenkamp, saying he mistook her for an intruder at his home.

At the original trial in September last year, Judge Thokozile Masipa ruled that the state had failed to prove intent or "dolus eventualis", a legal concept that centres on a person being held responsible for the foreseeable consequences of their actions.

Dolus eventualis refers to whether a person foresees the possibility that his or her action will cause death but carries on regardless.

"In these circumstances, the accused must have foreseen and, therefore, did foresee that whoever was behind the toilet door might die but reconciled himself to that even occurring and gambled with that person's life," said Judge Leach.

"The identity of his victim is irrelevant to his guilt."