South African Olympic star Oscar Pistorius faced fierce cross-examination at his murder trial Wednesday, with the prosecution demanding he admit his girlfriend's death was more than just a "terrible mistake."

Prosecutor Gerrie Nel also told Pistorius that Reeva Steenkamp's head exploded when she was shot and tried to force him to look at a horrific police photograph of her injuries.

The athlete has tearfully testified that the 29-year-old model died in his arms, after he shot her mistaking her for an intruder on Valentine's Day last year.

"You made a mistake?" Nel thundered indignantly just moments after opening the state's cross-examination.

"You killed a person, that's what you did! You shot and killed her, won't you take responsibility for that?"

"Say it, say you killed a person!"

Judge Thokozile Masipa adjourned proceedings multiple times to allow Pistorius time to collect himself.

When Nel resumed he showed footage of the Olympian firing a bullet into a watermelon at a shooting range.

After seeing images of the fruit's pink flesh exploding through the air and hearing Pistorius remark it was "softer than brains" Nel turned to the witness box.

"You know that's the same that happened to Reeva's head," he said, causing a shocked Pistorius to grow more distraught.

Unrelenting, Nel then quickly showed the court the graphic photo of Steenkamp's bloodied head, her blonde hair drenched with dark coagulated blood.

"That's it," Nel said, turning to Pistorius. "Have a look there, I know you don't want to because you don't want to take responsibility."

Growing ever more emotional and affronted, Pistorius said: "My lady I've taken responsibility."

"I don't want to look at a picture where I'm tormented by what I saw," he said wailing through tears.

"I don't have to look at a picture, I was there," said Pistorius, burying his head into his hands.

"Stern test"

Pistorius's cross-examination is a key point in his trial, and a stern test of both his version of events and of his resolve.

During the five-week trial the world-famous athlete has appeared fragile, frequently crying in court and becoming physically sick when the gruesome details of Steenkamp's injuries were discussed.

In his later testimony on Wednesday Pistorius seemed in turn defiant and harrowed, refusing pleas to answer questions directly.

"I'm fighting for my life," he said desperately after being accused by Nel of lying and tailoring his answers.

Pistorius is likely to remain on the stand for the rest of this week as his extensive testimony is probed and picked at by Nel, who once won a corruption conviction against a South African police commissioner.

The 27-year-old Paralympian earlier wrapped up three days of emotional and harrowing defence testimony by denying he wanted to kill Steenkamp.

"I did not intend to kill Reeva, my lady, or anyone else," he told the murder trial, in his strongest rebuttal yet of charges against him.

He faces a life sentence if convicted of her murder.

But he could also face a murder conviction if the prosecution shows he intended to kill when his life was not threatened -- regardless of whether he knew who was behind the door.

"Remorse"

In calling Pistorius as a witness in his own defence, his handpicked legal team tried to counter the state's portrayal of him as reckless and obsessed with fast cars and guns.

Defence lawyer Barry Roux gently painted the portrait of a young man who likes dogs and carries out charitable acts, but who was marked by a fatherless childhood, the early death of his mother, physical disability and recurring crime.

Pistorius had described how he met Steenkamp, a vivacious law graduate, and how they quickly grew closer and planned their future together.

He demonstrated remorse by apologising publicly to Steenkamp's family for what happened, a display that could be presented as a mitigating factor in any sentencing.

He also gave horrifying testimony about his vain attempts to stem Steenkamp's blood loss and save her life using plastic bags and utility tape.

"I was really trying to stop the bleeding, I was really trying to help Reeva breathe," said Pistorius, still struggling to retain his composure.

"Reeva had already died while I was holding her," he said. "There was nothing more I could do for her."

Defence lawyer Roux is expected to call up to 15 more witnesses in the remainder of the case, to testify on ballistics, whether Steenkamp urinated, damage to the toilet door, sound, as well as Pistorius's fear of crime and "vulnerability" on his stumps.

Eventually set down for three weeks, the trial could run until mid-May, possibly even longer.

Pistorius's cross-examination will continue on Thursday.