The prosecutor in the South African murder trial of Oscar Pistorius ended his five-day cross-examination of the athlete with a summary of how he shot his girlfriend.

Prosecutor Gerrie Nel insisted Mr Pistorius killed Reeva Steenkamp deliberately after an argument.

He said: "You fired four shots through the door whilst knowing that she was standing behind the door," Mr Nel said.

"She was locked into the bathroom and you armed yourself with the sole purpose of shooting and killing her."

Mr Pistorius, 27, replied: "That is not true."

The athlete faces life in prison if convicted of murder.

Mr Pistorius has broken down in tears on many occasions during the questioning.

At one point he retched into a bucket on the witness stand after being shown grisly pictures of Ms Steenkamp after the shooting on Valentine's Day last year.

He insists he killed the 29-year-old law graduate and model accidentally after mistaking her for an intruder hiding behind a closed toilet door.

Today, Mr Pistorius told the court he had pulled the trigger without thinking after hearing a noise behind the door, out of terror and fear that his and Ms Steenkamp's lives were in danger.

"I was extremely fearful, overcome with a sense of terror and vulnerability," said Mr Pistorius.

"I didn't think about pulling the trigger, as soon as I heard the noise, before I could think about it, I pulled the trigger."

The athlete's voice quivered as he recounted how he was "overcome with terror and despair" on finding Ms Steenkamp’s bloodied body slumped against the toilet after he broke down the door with a cricket bat.

"I was broken, I was overcome, filled with sadness," he told judge Thokozile Masipa, adding he urged Ms Steenkamp to hold on while he sought help from neighbours at his high-security Pretoria residence.

Mr Pistorius insists he and Ms Steenkamp were in a loving, if fledgling, relationship, despite phone text messages read in court which pointed to some arguments.

Today in court, he read a Valentine's Day card his girlfriend got for him before her death.

"Roses are red, violets are blue," the card begins.

"I think today is a good day to tell you that I love you," the message concludes, the last part in Ms Steenkamp's own words.

The defence later moved onto questioning its third witness, with the trial looking likely to run into next month.

The murder trial has captivated South Africa and millions of athletics fans around the world who viewed Mr Pistorius, known as the 'Blade Runner' because of the carbon-fibre prosthetics he used on the track, as a symbol of triumph over adversity.

His disabled lower legs were amputated as a baby but he went on to achieve global fame, winning Paralympic gold medals and reaching the semi-finals of the 400 metres in the 2012 London Olympics against able-bodied athletes.