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Pistorius is accused of shooting dead 29-year-old Steenkamp following a heated row in the early hours of Valentine's Day last year. The track star says he killed Steenkamp after mistaking her for an intruder hiding in the toilet.
"During an adjournment in court today, my client Kim Myers was approached by Oscar Pistorius who whispered in a sinister undertone: 'How can you sleep at night?" Myers' lawyer Ian Levitt told Reuters. "She is horrified by this disturbing act but will not be intimidated and intends to carry on attending the trial for Reeva."
Local media reports said Pistorius denied speaking to Myers when asked about the alleged incident at the court by reporters.
A reason for what Levitt said was Pistorius' attempt to intimidate Myers was unclear. She has regularly attended the trial and has been quoted in the media commenting on her close friendship with Steenkamp.
The state prosecution has spent much of the 27-day trial trying to portray Pistorius as an intimidating figure who bullied Steenkamp, although the Paralympic gold medallist insists they were in a loving relationship.
Phone text messages sent between the couple were read out to the court earlier in the trial, one of which from Steenkamp said she was scared of Pistorius, although most of the retrieved conversations were loving.
The trial of Pistorius, who faces life in prison if found guilty, has attracted huge interest across the world.
Before the shooting, Pistorius was one of the most revered figures in sport, a man who had his lower legs amputated as a child but who reached the semi-finals of the 400m at the London Olympics in 2012 running on carbon-fibre prosthetic "blades".