The father of a black teenager shot dead in by a U.S. neighbourhood watch volunteer said Monday he never denied it was his son's voice screaming for help on a phone call to police, contradicting officers' testimony at George Zimmerman's murder trial.

Also Monday, the judge ruled that defence attorneys may present evidence that the teen, Trayvon Martin, had marijuana in his system when he died.

Convincing the jury of whose voice is on the phone tapes is important to both sides because it would help jurors decide on the aggressor in the nighttime confrontation.

The case drew national protests when Zimmerman went without arrest for weeks, and questions have focused on race and self-defense gun laws.

Zimmerman has pleaded not guilty to second-degree murder and says he shot Martin in self-defense during a scuffle in the gated community where he lived. Martin was there visiting family.

Prosecutors contend that Zimmerman was profiling Martin and perceived the teen as someone suspicious in the neighbourhood, which had been the site of a series of break-ins.

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The parents of Trayvon Martin, Tracy Martin, right, and Sybrina Fulton, listen to the testimony of Sanford police officer Chris Serino during the George Zimmerman trial in Seminole Circuit Court, in Sanford, Fla., Monday, July 8, 2013. Zimmerman is charged with second-degree murder in the fatal shooting of Trayvon Martin, an unarmed teen, in 2012. (Joe Burbank, Orlando Sentinel/Associated Press)

The lead detective in the case, officer Chris Serino, spoke about a meeting with Martin's father in the days after the black teen was fatally shot by Zimmerman last year.

Serino testified that when Tracy Martin listened to the police recording and was asked if it was his son, Martin said "no," Serino said.

"He looked away and under his breath he said `no'," Serino said.

Under cross examination, prosecutor Bernie de la Rionda suggested that Martin may have been in denial about his son's death.

"It could be perceived as denial," Serino said.

The teen's father denied the officer's account. "I never said that wasn't my son's voice," said Tracy Martin, who added that he concluded it was his son after listening to the call as many as 20 times.

The emergency call captured the confrontation between Zimmerman and Martin shortly before Zimmerman fatally shot the teen. Zimmerman's mother and uncle have testified it was Zimmerman screaming. Martin's mother and brother said the voice belonged to Martin.

Zimmerman himself once said during a police interview that the screams didn't sound like him, though he and his family later said the screams were his.