His voice cracking with emotion, Oscar Pistorius took the witness stand in his own defense on Monday, saying the Valentine's Day shooting of his girlfriend last year had left him sleepless, terrified and plagued by nightmares.

The disabled South African track star, on trial for murder, also apologized to the mother of model Reeva Steenkamp, saying he fired four times through a toilet door at his luxury Pretoria home in the belief he was defending her from an intruder.

Steenkamp, a law graduate and model, was hit by at least three rounds fired by Pistorius, one of which killed her almost instantly, the court has heard.

"I was simply trying to protect Reeva," Pistorius told the court at the start of his testimony.

Since the shooting, 27-year-old Olympic and Paralympic star Pistorius, who faces life in prison if convicted of murder, testified he had been on anti-depressants and sleeping pills because of his disturbed state of mind.

"I'm scared to sleep. I have terrible nightmares," he said. "I can smell blood. I wake up to being terrified.

Reuters