Defence lawyers have released a video showing neighbourhood watch volunteer George Zimmerman re-enacting for police the fight he says led him to fatally shoot black Florida teen Trayvon Martin.

In the video, accompanied by audio and written statements, Zimmerman said he only shot the unarmed 17-year-old after Martin slammed his head repeatedly into the sidewalk, saw his gun and told him "you're gonna die tonight".

Zimmerman (28) is charged with second degree murder over the 26 February shooting in Sanford, Florida and faces life in prison in a racially-charged and high-profile case that sparked protests across the United States.

There have been two starkly different versions of the incident.

Prosecutors say Martin was simply "minding his own business," returning home from buying an iced tea and some Skittles from a local store, when he was accosted by Zimmerman and shot dead.

Zimmerman told police he was tracking Martin after viewing him as suspicious, but shot and killed the teen purely in self-defence after being assaulted by him.

Taken a day after the shooting, the video shows Zimmerman, who was not to be arrested for several weeks, walking around the Sanford housing estate where the shooting occurred giving the police his version of events.

With two butterfly bandages on the back of his head and apparent bruises on his face, Zimmerman says Martin punched him in the face and started "slamming and slamming" his head on the pavement.

Zimmerman says that when his jacket slid up and revealed his holstered pistol: "He (Martin) reached for it. I felt his arm going down by my side. I just grabbed my firearm and I shot him. One time."

In a written statement, Zimmerman said he had broken off his pursuit and was returning to his car when Martin "emerged from the darkness and said, 'You got a problem?'"

"I said, 'No.' The suspect (Martin) said, 'You do now!'" Zimmerman said. While he then fumbled for his cellphone to call police, Martin punched him in the face.

"I fell backward onto my back. The suspect (Martin) got on top of me. I yelled, 'Help' several times. The suspect told me to 'shut the f**k up.'

"As I tried to sit upright, the suspect grabbed my head and slammed it into the concrete sidewalk several times... My head felt like it was going to explode," Zimmerman said.

He said he tried to slide out from under Martin while still screaming for help, then Martin tried to "smother" his mouth and nose and reached for Zimmerman's gun and said: "Your gonna (sic) die tonight mother f**ker."

Claiming he feared for his life, Zimmerman said he unholstered his gun and "fired one shot into his torso".

"You got me," Martin said as he fell back, according to Zimmerman's account.

The case caused consternation, mainly over authorities' initial reluctance to press charges against Zimmerman, who insists that he acted in self-defence and within his rights under Florida's controversial stand-your-ground law.

Zimmerman, whose father is white and whose mother is Hispanic, was eventually arrested on 11 April and charged with second-degree murder.

Sanford police chief Bill Lee was fired on Wednesday. A statement said Lee, who took a leave of absence in March, had lost "the trust and respect of the elected officials and the confidence of the entire community".

Zimmerman was initially released on $150 000 bail but ordered back to jail earlier this month after being found to have misled the court about his finances. Court officials have set a new bond hearing for 29 June.