The chief prosecutor in the Oscar Pistorius murder trial has told the athlete that he shot his girlfriend through a toilet door as they were talking.

Gerrie Nel said Pistorius intentionally killed Reeva Steenkamp and put it to the double-amputee Olympian during cross-examination that "she was standing behind the toilet door talking to you when you shot her".

Pistorius said "that's not true" in a dramatic exchange to end the first week of the runner's evidence at his trial.

Mr Nel said the "only reasonable explanation" for the shooting last year is that Pistorius killed the model after a fight.

Pistorius claims he shot Steenkamp by mistake thinking she was a night-time intruder behind a toilet door in his bathroom.

Mr Nel accused Pistorius of shooting Steenkamp through the toilet door with his 9mm pistol as the couple argued in the early hours of Valentine's Day last year.

Pistorius said he shouted at what he thought was an intruder in his house and also at Steenkamp to call the police.

Mr Nel said that if that were the case, she would not have stood up against the door. She would have retreated away from it and she would have responded to Pistorius, the chief prosecutor said.

"I don't think anybody could say where she would have stood," Pistorius replied.

The double-amputee Olympian is charged with premeditated murder. He claims he shot the model and rising reality TV star by mistake, thinking she was an intruder about to come out the toilet and attack him.

Mr Nel led the double-amputee runner through his own account of what happened in the moments before he shot Steenkamp.

Pistorius said he heard a noise in the bathroom and moved down a hallway on his stumps towards the bathroom while screaming to his girlfriend - who he claims he had believed was in the bedroom - to get down and call the police.

He said he then heard what sounded like the toilet door slamming, then kept quiet as he reached the bathroom entrance, then heard a noise in the toilet that he perceived to be the sound of wood on wood, which he said made him think someone was opening the toilet door - which he said fit badly in the frame - to attack him. And then, Pistorius said, he opened fire.

At each stage, Mr Nel argued that the account was improbable, questioning why Pistorius did not establish where Steenkamp was and make sure she was okay, and why he would approach the alleged danger zone if he felt vulnerable on his stumps.

Mr Nel said: "If you spoke to Reeva, the two of you could have taken lots of other steps."

Pistorius said he thought the perceived threat could strike at any moment: "There was no time."

Mr Nel also highlighted what he said was an inconsistency in Pistorius's evidence, noting that the Olympic runner had said Steenkamp did not scream when he opened fire but also saying he could hear very little because his ears were "ringing" from the first gunshot.

"You knew that Reeva was behind the door and you shot at her knowing that she was behind that door," Mr Nel said.

"That's not true, my lady," Pistorius said, addressing the judge.