Oscar Pistorius, Reeva Steenkamp 'argued' on night she died

Oscar Pistorius argued with his girlfriend shortly before shooting her dead, the prosecution lawyer alleged on Monday as his gruelling cross-examination of the sprinter went into a second week.

Prosecutor Gerrie Nel accused the 27-year-old of concocting evidence in his defence against the charge of murdering Reeva Steenkamp on Valentines' Day last year.

"It's the state's case, Mr Pistorius, that she wanted to leave and that you weren't sleeping, you were both awake," said Nel. (Pistorius tailored evidence: Prosecution)

"That's not correct my lady, that's untrue," Pistorius replied softly. "There was an argument," Nel said, drawing another denial. Nel repeatedly described the athlete's explanation that he accidentally shot Steenkamp through a locked bathroom door as "so improbable that it cannot be reasonably possible".

The prosecutor's relentless questioning has elicited tears and taut replies from the world-famous double amputee, who insists he and the 29-year-old model were in a loving relationship.

During the relentless back-and-forth Pistorius and Nel at times appeared to come close to bickering. (Pistorius' version 'impossible': prosecutor)

Nel complained that Pistorius was being evasive and challenged him: "Today I pick up you're not sure about things, is anything wrong?" Nel asked. "You're fine?"

Pistorius grew increasingly restless during the morning, wiping his face, pinching the bridge of his nose and clenching his jaw.

Jumping from one event or piece of evidence to another, Nel sought to keep Pistorius off balance. (Pistorius admits

The athlete could not explain why the model and law graduate had undigested food in her stomach up to eight hours after the couple ate under his version of events.

A forensic pathologist earlier testified that she must have eaten around two hours before her death, which contradicts the accused's version they were sleeping at the time.

The defence has disputed the science that stomach contents could accurately indicated the time of the last meal.

The athlete was also asked to explain why Steenkamp's jeans were lying on the floor while the rest of her possessions were neatly arranged.

Nel has openly called the athlete's version "a lie" contending Pistorius knew exactly what he was doing when he fired the lethal shots.

"You knew Reeva was behind the door and you shot at her!" he said Friday, with athlete replying softly "It's not true, milady."

Pistorius claims he woke up in the early morning hours, brought in two fans from his balcony, then switched armed himself after hearing the noise in the bathroom.

Steenkamp's older sister branded Pistorius a "disgusting liar" after attending some of his testimony in court with her mother.

"He is trying to convince the court that they were really close and that he cared for her. It's not true," said Simone Steenkamp, 48.

Attending the trial "almost destroyed me", she told Britain's Mail on Sunday.

"He knew we were there but he almost lost his balance and fell over when he saw us.

"Then he sat in the dock, smirking. He killed my sister and yet he still seems to be enjoying his celebrity status.

"During the adjournments he marched around as if he was still a star while my mother and I just wanted to hide away," she added.

"I don't understand how anyone could commit that act - kill someone - and behave like that."

Pistorius will likely remain on the stand for another few days as a witness in his own defence.

His lawyers have said it will call up to 17 witnesses, including ballistics experts.

The double amputee gained world-wide fame for running on two carbon fibre blades at the Paralympics and 2012 London Games. His legs were amputated below the knee shortly after he was born without calf bones.

The shooting brought his career to an abrupt halt, but die-hard fans insisting on his innocence have attended the trial from time to time.

On Friday a fan handed him a bouquet of flowers as he came out of court, while on Monday three women outside the building held white balloons with "Oscar" and "Love" scribbled in permanent marker.

Originally set down for three weeks, the trial on Monday enters its sixth week and has been extended until May 16. (Pistorius admits 'no reason' to fire shots)