Oscar Pistorius was accused of making a "sinister" remark to a friend of Reeva Steenkamp in the courtroom during a break at his murder trial.
Pistorius denied the allegation that he said on Tuesday to the close friend of the girlfriend he killed: "How can you sleep at night?"
The accusation by Kim Myers provided a bizarre twist during the trial of the double-amputee Olympian, who is facing 25 years to life in prison if convicted of premeditated murder for shooting dead Steenkamp, a 29-year-old model, at his home last year.
Pistorius, 27, denies murder and says he killed Steenkamp by mistake thinking she was an intruder when he shot her multiple times through a closed toilet door.
He also denied to reporters in the courtroom that he made any comment to Myers. Pistorius said he hadn't spoken to members of the Myers family for weeks, despite sitting a few metres from them during proceedings. Myers' lawyer said she told him it happened. Myers was approached by Pistorius, and he made the remark to her in a "very sinister way", the lawyer for Myers said. The lawyer, Ian Levitt, said Myers was "shocked" and did not know what it referred to. Levitt said she found it "extremely disturbing".
"It's important to confirm this. It was in front of witnesses. Journalists have witnessed this," Levitt said of the alleged comment. Levitt was not in the courtroom but said Myers phoned him to report it.
Reporters said Pistorius apparently made a comment to Myers as he walked close to the court benches reserved for family and friends of both Pistorius and Steenkamp. Levitt added that he made a complaint to state prosecution officials, although officials at South Africa's National Prosecuting Authority said they would not be acting on the report.
The Myers family was close to Steenkamp. She lived with them after moving to Johannesburg to further her modelling career, and they referred to themselves as her second family. They have attended much of Pistorius' trial.