Oscar Pistorius has admitted he had "no reason" to fire the shots that killed his girlfriend as a prosecutor grilled him about another incident in which a gun went off in his possession.

The Paralympic sprinter insisted he killed Reeva Steenkamp by accident at his home on Valentine's Day last year, but could not explain why he pulled the trigger.

Pistorius, 27, spent a full day under gruelling cross-examination by prosecutor Gerrie Nel at the high court in Pretoria, South Africa. Pistorius claimed he thought he had heard an intruder, but repeatedly refused to say he intended to shoot anyone.

Nel challenged him: "We know for a fact there were no intruders in your house that night, we know for a fact there was no ladder against the wall. We know for a fact that you had no reason to shoot, objectively speaking."

Pistorius told the judge: "That's correct, my lady."

"So why shoot?" Nel asked. Pistorius explained: "I heard a noise coming from the toilet that I interpreted as someone coming out to attack me."

Nel pressed him: "Accidentally your fingers pulled the trigger?"

When Pistorius agreed, Nel said: "At the intruder?"

But Pistorius countered: "At the door."

As Nel became exasperated, Pistorius broke down, his voice shaking as he insisted: "It was an accident."

Earlier, Nel addressed another charge relating to an incident at a restaurant in Johannesburg in which a shot went off after a friend of the runner passed him a loaded gun under the table.

Pistorius insisted he did not have his finger on the trigger when the gun fired. The prosecutor noted that a police expert had testified that the gun could not be fired without pulling the trigger, and sarcastically described the discharge as a "miracle".

Nel asked: "The gun went off by itself?"

Pistorius was adamant: "I know that my finger was not on the trigger."

Nel, incredulous, went on: "We have you in possession of the gun, a shot went off, but you didn't discharge the gun."

Pistorius said: "I didn't pull the trigger. I didn't have time to think."

Nel mimicked him sarcastically: "I'm a gun enthusiast, I didn't have time to think."

Nel also grilled Pistorius on another of the charges against him, that the athlete had rounds of .38-calibre ammunition in a safe at his home without a licence. Pistorius pleaded not guilty to the charge and said it was his father's ammunition, put there for safekeeping.

Nel said Pistorius's father, Henke, had refused to make a statement to police about the ammunition. Henke was present at Pistorius's bail hearing, but has not attended the trial.

Nel, staring at Pistorius, told him: "You are lying. You just don't want to accept responsibility for anything."

He also forced Pistorius to admit that he sometimes illegally left a magazine of bullets in his bedside table, including on 13 February 2013, the day before the tragedy.

Nel pointed to police photos of Pistorius's bedroom and suggested they were inconsistent with the athlete's account of waking up, speaking to Steenkamp and getting up to bring in two fans. "When you got up you had an argument," Nel said. "That's why she ran away screaming."

The prosecutor referred to WhatsApp messages between Pistorius and Steenkamp which he said portrayed the athlete as self-centred and egotistical. He said the phrase "I love you" appeared only twice and, on both occasions, it was written by Steenkamp to her mother. "Never to you and you never to her," Nel said.

Pistorius responded: "I never got the opportunity to tell Reeva that I loved her."

Nel also accused Pistorius of ignoring the concerns of Steenkamp's family by apologising to them at the beginning of his testimony this week instead of doing so in private.

Nel said: "Why would you create a spectacle in court, in the public domain, in the public eye? Why did you put them through this?" He added, in a recurring theme of this cross-examination: "Your life is just about you … "You will blame anybody but yourself."