Merryll Vorster: Pistorius "is certainly remorseful about the events... and has developed a depressive disorder" Photo: BBC

His actions when he shot his girlfriend on Valentine's Day last year "should be seen in context of his anxiety," Merryll Vorster said.

The prosecution dramatically asked for him to undergo mental observation.

The double-amputee Paralympian denies intentionally killing Reeva Steenkamp.

'Fight rather than flight'

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AnalysisAndrew HardingAfrica correspondent

This was a day of unexpected drama in courtroom GD, with the possibility of a lengthy delay now hanging uncomfortably in the air.

Speaking in court immediately after the adjournment, Oscar Pistorius told me the prosecution's application to refer him for further psychiatric evaluation was "a joke" and insisted that today's evidence from Dr Merryll Vorster had "gone well".

But the course of this already extended trial now hinges on Judge Masipa's decision - likely on Tuesday - on whether to grant the prosecution's request.

The defence has always maintained that the athlete's disability and vulnerability would play an important role in their case - hence today's expert who diagnosed his "general anxiety disorder". It is now clear Mr Pistorius' team feel Prosecutor Gerrie Nel is "playing games".

But the state believes the psychiatrist's evidence is further proof that the athlete is changing his defence - from putative self-defence, to an accidental shooting, to something now linked to his state of mind.

From what I have gleaned from numerous sources in court, it is likely that Judge Masipa will not grant the prosecution its request, but we shall know for certain only when she announces her decision.

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The BBC's Milton Nkosi says it is as if tectonic plates are shifting at the trial as the prosecution moved towards getting permission from the judge for Mr Pistorius to be mentally assessed.

The defence opposed the application before the court adjourned on Monday. It will reconvene on Tuesday.

If the prosecution request is granted, Mr Pistorius may spend up to 30 days in a state mental health institution for observation and assessment of his mental health.

It was expected that the defence would conclude its case by the end of this week - after which both sides would have an opportunity to present their closing arguments.

Mr Pistorius says he accidentally shot Ms Steenkamp through the toilet door in a state of panic, mistaking the 29-year-old model and law graduate for an intruder.

Giving evidence on Monday, Dr Vorster said that Mr Pistorius was more likely to respond to any threat with "fight" rather than "flight".

The anxiety disorder was the result of surgery at the age of 11 months to remove his lower legs, she said, a "traumatic assault" for an infant at that age.

She said that Mr Pistorius felt remorse over Ms Steenkamp's death.

"He feels guilty and has developed a depressive disorder as a result," she said.

Family members of South African Paralympic athlete Oscar Pistorius attended his trial on Monday

The couple had been dating for just a few months when she was shot dead in 2013

The psychiatrist said that the reactions of Mr Pistorius in the early hours of 14 February 2013 would have been different to that of a "normal, able-bodied person without generalised anxiety disorder".

However, she said that this would not have affected his ability to distinguish between right and wrong and that it was up to the court to decide whether his anxiety disorder - from which he had suffered since childhood - diminished his responsibility.

"I think the generalised anxiety is relevant to the case. But the court will have to decide," she said.

'Danger to society'

Dr Vorster said generalised anxiety disorders are not uncommon, and were not signs of mental illness.

Safety measures at his home were "out of proportion" to the threat of crime in South Africa, she said.

She said that Mr Pistorius' parents separated when he was six and his father was not a responsible parent.

"[He was] largely absent, and his mother was anxious, sleeping with a firearm under her pillow," she said.

She said that his mother's death in March 2002 meant that he lost an "emotional attachment figure".

The order and trajectory of bullets that killed Reeva Steenkamp is a key part of the case

Mr Pistorius suffered bouts of anxiety as child because of his parents' separation and his disability, a psychiatrist said

State prosecutor Gerrie Nel says that Mr Pistorius shot Reeva Steenkamp dead after a row

State prosecutor Gerrie Nel asked Ms Vorster whether someone with anxiety disorder plus guns would be "a danger to society".

"Yes," she replied.

There are no juries at trials in South Africa, so the athlete's fate will ultimately be decided by Judge Thokozile Masipa, assisted by two assessors.

If found guilty, Mr Pistorius - a national sporting hero dubbed the "blade runner" because of the prosthetic limbs he wears to race - could face life imprisonment.

If he is acquitted of murder, the court must consider an alternative charge of culpable homicide, for which he could receive about 15 years in prison.