South African runner Oscar Pistorius says he did not intend to kill his girlfriend, Reeva Steenkamp, or anyone else the night he shot her at his Pretoria home last year.

In his third day of testimony Wednesday, Pistorius told the court he felt "helpless" while waiting for an ambulance to arrive, but that by the time paramedics came it was too late.

"Reeva had already died while I was holding her, before the ambulance arrived, so I knew that there was nothing that they could do for her."

Pistorius has told the court he thought he was shooting at an intruder when he fired through a locked bathroom door.

Prosecutors say Pistorius intentionally shot Steenkamp.

In his first responses to the prosecutor's cross-examination Wednesday, Pistorius said he "made a terrible mistake" in taking Reeva's life.

He has pleaded not guilty to charges of pre-meditated murder and three firearm-related offenses. If convicted, he faces 25 years to life in prison.

Pistorius is known as the "blade runner" for his prosthetic legs. He became the first double-amputee to compete in the Olympics when he ran at the 2012 games in London.