A key witness in the trial of the Minneapolis police officer accused of murdering George Floyd sought Tuesday to avoid giving evidence as his lawyers said he could incriminate himself on separate charges.

Morries Hall, who was with Floyd in his car shortly before his death, is in custody and appeared by video camera at the hearing after he was subpoenaed to testify.

"There's really a very small narrow topic that might be permissible," Judge Peter Cahill said as Hall's lawyers argued that it was impossible for him to be cross-examined without incriminating himself.

Hall is seen as a potentially important witness for the defense of former police officer Derek Chauvin, who was seen in a phone video kneeling on the neck of a handcuffed Floyd for more than nine minutes.

The harrowing footage of Floyd's May 25, 2020 arrest touched off protests against racial injustice and police brutality in the United States and around the world.

Minneapolis police chief Medaria Arradondo testified on Monday that Chauvin violated training policies by kneeling on Floyd's neck after he had stopped resisting.

Further witnesses on Tuesday largely took the same line as they were questioned over Chauvin's actions.

Flags and flowers adorn "George Floyd Square" in Minneapolis, Minnesota as the trial over his death continues
Flags and flowers adorn "George Floyd Square" in Minneapolis, Minnesota as the trial over his death continues AFP / Kerem Yucel

Shown a picture of Chauvin kneeling on Floyd's neck, Lieutenant Johnny Mercil, the police's use-of force coordinator, said it was not an authorized restraint.

"We tell officers to stay away from the neck when possible. And if you're going to use body weight, to pin it on their shoulder," Mercil said during cross-examination.

Prosecutors are seeking to prove that Floyd's death was due to asphyxiation, while Chauvin's defense claims it was due to illegal drugs in Floyd's system.

Chauvin's lawyer Eric Nelson confirmed he wanted to ask if Hall sold or gave Floyd drugs.

Hall's lawyer said "there's an allegation here that Mr Floyd ingested a controlled substance as police were removing him from the car... This leaves Mr Hall potentially incriminating himself."

The judge said he would rule later on Hall's request not to testify.

Chauvin, who was sacked from the police force after the incident, has pleaded not guilty to charges of murder and manslaughter.