South Africa's appeal court on Friday ruled that paralympian Oscar Pistorius - previously sentenced to six years in jail for murder - should receive a harsher prison term, increasing his sentence to 13 years and five months.

"The appeal against sentence is upheld ... The respondent is sentenced to imprisonment for a period of 13 years and five months," judge Willie Seriti said on Friday, followed by audible gasps from the audience in the courtroom.

The state prosecution had appealed the sentence earlier this month, saying that Pistorius' punishment was too lenient.

Pistorius has served just over a year of that six-year sentence.

The minimum sentence for murder in South Africa is 15 years.

In 2015, in a case that gripped South Africa and the world, the 30-year-old sprinter was found guilty of the Valentine's Day murder of his model girlfriend Reeva Steenkamp.

The prosecution argued that Pistorius' disability should not have been a mitigating factor and that the athlete, who wept regularly throughout the trial, had not shown genuine remorse, but rather felt sorry for himself and regretted his actions.

The South African double-amputee sprinter, known as the "Blade Runner," maintains he believed Steenkamp was an intruder when he shot her through the bathroom door.

It is the second time the prosecution is appealing. Pistorius was originally found guilty of culpable homicide, but this was changed to murder on appeal.

Pistorius, whose lower legs were amputated because of a congenital defect, soared to international fame after becoming the first amputee to compete in the 2012 Olympic Games.