South African Olympic and Paralympic track star Oscar Pistorius has taken the witness stand in his own defence at his murder trial, saying the shooting of his girlfriend had left him sleepless, terrified and plagued by nightmares.

At the trial on Monday, he also apologised to the mother of Reeva Steenkamp in the public gallery, saying he fired four shots through a toilet door in the early hours of Valentine's Day last year in the belief that he was defending her from an intruder, Reuters news agency reported.

"I was simply trying to protect Reeva," he told the court, his weeping voice quavering with emotion.

Pistorius, 27, faces a life sentence in prison if convicted for killing his girlfriend. He has pleaded not guilty to charges of murder and three firearms-related offences.

He said he is taking anti-depressants and sleeping pills.

Prosecutors say Pistorius killed Reeva following an argument, a charge that he denies.

Source: Agencies