JOHANNESBURG (AFP) - South Africa has put the release of Paralympian Oscar Pistorius on hold, saying a decision to free him on Friday after serving only 10 months in prison for killing his girlfriend had been premature.

Justice Minister Michael Masutha said in a surprise statement on Wednesday that the June decision to allow Pistorius to serve out the remainder of his five-year sentence under house arrest had "no legal basis" and was suspended pending a parole board review.

The 28-year-old athlete was convicted in October of culpable homicide - a charge equivalent to manslaughter - over the killing of his girlfriend Reeva Steenkamp on Valentine's Day in 2013, but prosecutors have appealed, seeking a murder conviction instead.

Wednesday's announcement comes on the very day Steenkamp, a model and law graduate, would have celebrated her 32nd birthday.

"It is apparent... that the decision to release him on 21 August 2015 was made prematurely on 5 June 2015, when the offender was not eligible to be considered at all," Masutha said in a statement.

"The earlier decision of the CSPB (the Correctional Supervision and Parole Board) to place the offender under correctional supervision is suspended until the Parole Review Board has decided on the matter," the statement said.

It was not immediately clear when the review board would meet and justice ministry officials were tight-lipped.

"It will depend on how and when the review board is going to hear the matter," ministry spokesman Mthunzi Mhaga told ENCA news channel, refusing to speculate about the possible date of his release.

The athlete's family said they were disappointed.

"We are shocked and disappointed that Oscar won't be home this Friday," a family member who did not want to be named told Reuters on Wednesday.

Steenkamp's parents June and Barry had been outraged by the idea of Pistorius's release so early.

During his high-profile and often emotional trial, Pistorius did not dispute that he shot Steenkamp four times through a locked toilet door in his upmarket Pretoria home in the early hours of Feb 14, 2013.

But he says he mistook her for an intruder.

Prosecutors, however, insist that he deliberately killed her after an argument, and this week appealed the culpable homicide verdict.

If they win their case, which is expected to be heard in November, Pistorius could face at least 15 years in jail.

He began serving his five-year sentence in October 2014 and will have served a sixth of it by Aug 21.

But the justice ministry said the initial release decision should not have been taken before he had actually served out that 10-month period.

The athlete - known as the "Blade Runner" for the prosthetic legs he wears on the track - won international fame after racing against able-bodied competitors in the 2012 London Olympics.

Once a poster boy for sport, Pistorius - whose legs were amputated below the knee when he was 11 months old - has lost his lucrative contracts and has no immediate hope of salvaging his athletic career whatever the outcome of the appeal.

His trial had exposed his darker side: offering glimpses of a dangerously volatile man with a penchant for guns, beautiful women and fast cars.

Criminal lawyer David Dadic said the initial parole board decision would have meant Pistorius having his prison sentence converted to house arrest.

While there had been speculation he would serve his house arrest at his wealthy Uncle Arnold's house - a mansion in a posh Pretoria suburb - there was a possibility the athlete would have asked to stay in a different location, away from the glare of the media.

Correctional services head Zach Modise said Pistorius, who is behind bars in Pretoria's Kgosi Mampuru II prison, initially wrestled with his sentence, but then committed to reform.

"At the beginning he could not understand that you get locked up in a cell. He struggled with that," Modise told South Africa's Sunday Times earlier this month.

"I think he's getting to understand you have to control your anger and temper," said Modise.

"I hope when he gets released on probation he will be able to conduct himself well."

On Wednesday, the Steenkamp family visited a beach in their home town of Port Elizabeth and threw roses into the sea at an intimate ceremony to mark Reeva's 32nd birthday.

"For our beautiful daughter - for anyone's life - it's definitely not long enough," Steenkamp's mother had previously told You Magazine, a South African tabloid, referring to Pistorius's sentence.

"She was robbed of her future, her career, her chance to get married and have a baby."