The trial of Oscar Pistorius has heard evidence from one of the first people on the scene after the shooting of Reeva Steenkamp.

Neighbour and doctor Johan Stipp told the court he feared the South African Olympic and Paralympic athlete might kill himself with the same gun.

Mr Pistorius denies murdering his girlfriend on St Valentine's Day 2013, arguing that it was a tragic mistake and that he mistook her for an intruder.

Testifying on the fourth day of Mr Pistorius's trial, Dr Stipp said he entered the athlete's home a few minutes after hearing screams and shots.

He said he found a distraught Mr Pistorius kneeling over the lifeless body of a woman.

Dr Stipp quoted Mr Pistorius as saying: "I shot her. I thought she was a burglar and I shot her."

Dr Stipp went on to describe his futile attempts to revive Ms Steenkamp, a 29-year-old model and law graduate who had been dating Mr Pistorius for a few months.

She died after being hit by three rounds, including one to the head, out of four fired by Mr Pistorius through the locked door of an upstairs toilet.

As Dr Stipp checked Ms Steenkamp for signs of life, Mr Pistorius was begging him to save her life, Dr Stipp told the court.

"Oscar was crying all the time. He prayed to God: 'Please let her live, she must not die'," he said.

At one point, when Mr Pistorius left Ms Steenkamp, Dr Stipp and housing complex manager Johan Stander to go upstairs, Dr Stipp thought he might be about to kill himself.

"I noticed that Oscar was going upstairs and I asked Mr Stander if he knew where the gun was because it was obvious that Oscar was emotionally very, very upset," he said.

"I didn't know the situation in the house so I thought maybe he was going to hurt himself."

If found guilty of intentional murder, Mr Pistorius is likely to spend at least 25 years in prison.