Oscar Pistorius has been jailed for five years for killing his girlfriend, Reeva Steenkamp, on Valentine's Day last year.
His defence team said he would spend a sixth of the term in prison - 10 months - before being placed under house arrest for the remainder of his sentence.
But Nathi Mncube, a spokesperson for the National Prosecuting Authority, said he was likely to serve one third of the sentence behind bars, before adding "it's not for the NPA to decide".
"This is not what we were hoping to achieve at the end of this trial ... but it is not entirely up to us," he told Sky News.
The 27-year-old, who was found guilty of culpable homicide last month, appeared to wipe his eyes as the sentence was handed down.
Video: Defence Expects Ten Months In Jail
Judge Thokozile Masipa had ruled the Olympic and Paralympic athlete did not intend to kill his girlfriend when he fired through the bathroom door at his home in Pretoria.
"Having regard to the circumstances in the matter, I am of the view that a non-custodial sentence would send the wrong message to the community," she said.
"On the other hand, a long sentence would also not be appropriate either, as it would lack the element of mercy.
Video: Sentence Disappoints Prosecutors
"What may appear to be justice to the general public may not be justice. Society cannot always get what they want."
She said Pistorius' skill using a gun was an aggravating factor but that she also recognised his remorse.
She said he "fired not one shot but four shots into the door" of a small cubicle "with no room for escape".
Video: Friends 'Felt Reeva's Light Today'
He was also handed a suspended three-year prison sentence for the discharging of a firearm, over an incident in which a gun accidentally went off in a restaurant.
Pistorius is expected to spend his jail time in the hospital wing of Pretoria's prison.
Ms Steenkamp's father, Barry Steenkamp, said he was "satisfied" with the sentence.
Her mother, June Steenkamp, said the sentence brought a sort of closure, but that full closure was not possible "unless you can magic her back".
Sentencing options available to the judge in the culpable homicide case included jail, a suspended sentence, a correctional supervision or fine.
It brings to a close a seven-month trial, which attracted worldwide attention.
Video: Pistorius Leaves Court For Prison
One of the most sensational trials in the country's history, the sentence is expected to fuel further controversy about race and money in its justice system.
Pistorius' legal team argued during the marathon trial that he mistook Ms Steenkamp for an intruder and believed they were both in danger.
His lawyer Barry Roux said the athlete had "lost everything" after the 29-year-old model's death, and argued he should not go to prison.
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Gallery: Reeva Steenkamp's Life In Pictures
Reeva Steenkamp, 29, was born in Cape Town and grew up in Port Elizabeth. She went to a convent school and studied law. She was a keen horse rider until she broke her back
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She moved to Johannesburg from Cape Town to model for Avon cosmetics. In 2012, Reeva was voted number 45 in the South African FHM 100 Sexiest Women in the World poll
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She featured as a celebrity contestant on BBC Lifestyle show Baking Made Easy in 2012
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The model was a keen Twitter user, and had more than 34,000 followers. She used the site to promote women's rights and empowerment
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Her former fashion editor, Barbara Robertson, described the model as being "sweet and down to earth" with the "it factor". She likened her to an "early Kate Moss"
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Reeva Steenkamp on the set of reality TV show Tropika Island Of Treasure (Pic: Stimulii)
He also said the Pistorius, whose lower legs were amputated as a baby, would face particular difficulties in jail.
But state prosecutor Gerrie Nel had argued that only 10 years' imprisonment would satisfy the South African public.
He said that any lesser sentence could result in a widespread loss of faith in the justice system.
Video: Pistorius Led Down To The Cells
Known as 'Blade Runner' because of his carbon-fibre prosthetics, Pistorius achieved global recognition at the London 2012 Olympics when he reached the semi-finals of the 400m against able-bodied athletes.
:: Watch Sky News for all the developments as they happen in Pretoria live on Sky channel 501, Virgin Media channel 602, Freeview channel 132 and Freesat channel 202.
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Gallery: Pistorius: A Sporting Pioneer
Pistorius was born in Johannesburg on November 22, 1986. A congenital condition meant he had no bones in his lower legs.
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He had both legs amputated below the knee when he was 11 months old, and now runs on carbon fibre prosthetic blades.
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Pistorius won the 200m final in a record time at the 2004 Athens Paralympics. In 2006 he finished sixth in the able-bodied event in South Africa.
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Ahead of the 2008 Beijing Olympics, the Court of Arbitration for Sport ruled his blades did not give him an unfair advantage. But he failed to qualify for the men's 400m.
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He won gold in all three of his events at the 2008 Beijing Paralympics - the 100m, 200m and 400m. He set a world record in the 400m (sport class T44).
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He won the 400m in 47.28s and the 100m in 11.04s at the 2011 Paralympic World Cup. The athlete later ran the 400m in 45.07s - within the Olympic qualifying time.
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The Blade Runner won the 400m silver medal in 45.52s at the 2012 African Championships. He just missed the 45.30s he needed to qualify for the Olympics.
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Pistorius was selected by the South African Olympic Committee to run the 4x400m relay and 400m at the 2012 London Olympics. He said it was "one of the happiest days of my life".
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He had been dating the model Reeva Steenkamp since late 2012. She was found dead at his home on February 14, 2013.
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After a lengthy trail, Pistorius is cleared of murder but found guilty of manslaughter and sentenced to five years in prison.
