Paralympic champion Oscar Pistorius has told the High Court in Pretoria that he has struggled to sleep ever since he shot dead his girlfriend Reeva Steenkamp.

The double amputee has taken to the stand to give evidence for the first time in his murder trial after the defence opened their case this morning following a week-long adjournment.

The 27-year-old denies murder by maintaining that he mistook the South African model for an intruder when he fired four gunshots through the bathroom door of his home on Valentine's Day last year.

Pistorius began his testimony in court with an apology, in which he said in a trembling voice: "I would like to take this opportunity to apologise to Mr and Mrs Steenkamp, to Reeva's family, to those of you who knew her and are here today."

He went on to say that he has been taking anti-depressants and sleeping pills since the shooting and has not slept properly for a number of weeks.

Pistorius told the court: "I am scared to sleep. I have terrible nightmares about what happened that night. I wake up and I can smell blood."

The athlete was the second defence witness to take to the stand following pathologist Professor Jan Botha.