Oscar Pistorius' prison sentence for murdering his girlfriend, Reeva Steenkamp, has been increased to 13 years and five months by a South African court.

The Olympic and Paralympic sprinter was sentenced to six years in July 2016 for shooting Steenkamp dead on Valentine's Day three years earlier.

Pistorius initially was given a five-year sentence after being convicted of culpable homicide before the conviction was upgraded to murder last year.

Prosecutors argued that he should have been given a longer prison term and they got their wish, as his sentence was more than doubled in a Supreme Court of Appeal judgment Friday. 

Pistorius, 31, was given a 15-year sentence — the minimum term for murder in South Africa — minus the time he has already served.

He has always maintained that he shot Steenkamp by mistake, believing her to be an intruder at his home in Pretoria.