MIAMI (AFP) - Three Florida police officers have been fired and a fourth resigned following an exchange of racist texts and an offensive video, officials said.

The episode comes at a critical moment in the United States as officers across the nation have come under fire for use of excessive force and allegedly targeting African Americans.

"All four officers' conduct involved racist text messages exchanged among themselves and former police officers," Fort Lauderdale Police Chief Frank Adderley said.

The offending materials included text messages with racist slogans and a pretend "movie trailer" made by one of the agents.

The clip featured a person wearing the white-robed outfit of a white supremacist Ku Klux Klan member and a photomontage of US President Barack Obama shown with gold-capped teeth.

An investigation was first initiated in October, at which point the four officers were dismissed from their normal duties.

Since then, one resigned and the other three were fired, Adderley said, announcing the results of the investigation.

"Their conduct was inexcusable and there is zero tolerance for this kind of behaviour in the Fort Lauderdale Police Department," Adderley said.

Fort Lauderdale Mayor Jack Seiler said he was "very disappointed, disgusted and shocked." The police force in the southeastern Florida city has around 500 agents.