George Zimmerman (C) and his attorney Mark O'Mara (R) in court

A Florida judge on Wednesday rejected a motion asking him to step down from the murder trial of George Zimmerman, the neighbourhood watch captain who killed unarmed black teenager Trayvon Martin, ruling the defence request was "legally insufficient."

Zimmerman's lawyer, Mark O'Mara, in a motion filed July 13 asking the judge to step down, accused Judge Kenneth Lester of bias, citing what he called "gratuitous" and "disparaging remarks" Lester made in a July 5 ruling raising Zimmerman's bond from $150,000 to $1 million, reports Reuters.

The ruling followed a bond hearing held after prosecutors accused Zimmerman and his wife Shellie, who has been charged with perjury, of lying to the court about their finances.

In his order dismissing the motion for him to recuse himself, Lester wrote that, by law, he must assume O'Mara's allegations were true.

"The court is not permitted to deny the allegations supporting the motion as untrue, reject them as unfounded, or comment upon them at all," Lester wrote in his order.

He went on to rule that despite the factual basis of the allegations they were not sufficient to require that he recuse himself. "The court finds the motion to be legally insufficient," Lester wrote.

In his July 5 order raising Zimmerman's bond, Lester rejected arguments by O'Mara that Zimmerman posed no risk to the community and his portrayal of the case against Zimmerman as weak.

"Under any definition, the defendant has flouted the system," Lester wrote at the time. He said Zimmerman's "stories changed with each retelling."

Lester also said O'Mara attempted to portray Zimmerman as a confused young man who "experienced a moment of weakness" and may have acted out of a sense of betrayal by the justice system.

"This court finds the opposite. The defendant has tried to manipulate the system when he has been presented the opportunity to do so," Lester wrote.

Zimmerman, a 28-year-old who is white and Hispanic, shot and killed Martin, 17, in a gated community in Sanford, Florida, where Martin was visiting his father. The black teen was unarmed and walking back from a store when Zimmerman called a 911 dispatcher and said the teen looked suspicious.

Zimmerman said he shot Martin in self defence after Martin attacked him and repeatedly slammed his head to the ground.

Martin's killing drew national attention after police initially declined to arrest Zimmerman, citing Florida's "Stand Your Ground" self-defence law and his assertion that he used deadly force because he feared his life was in danger.