A respiratory expert has testified that George Floyd died from lack of oxygen and police officer Derek Chauvin's knee was on his neck "more than 90% of the time" that he was handcuffed, face down in the street.

Dr Martin Tobin, an Irish-born pulmonologist, told the jury at Mr Chauvin's murder and manslaughter trial that he had watched videos of Mr Floyd's 25 May 2020 arrest "hundreds of times".

"Mr Floyd died from a low level of oxygen," Dr Tobin told the jury hearing the high-profile case in a heavily guarded Minneapolis courtroom.

"This caused damage to his brain," he said, and arrhythmia - an irregular heartbeat - which "caused his heart to stop".

Mr Chauvin, 45, who is white, was seen in a video taken by a bystander kneeling on the neck of a handcuffed Mr Floyd for more than nine minutes as the 46-year-old African-American man complained repeatedly that he "can't breathe".

The video of Mr Floyd's arrest touched off protests against racial injustice and police brutality in the United States and around the world.

Dr Tobin said Mr Floyd's breathing weakened because he was face down on the street, handcuffed and with Mr Chauvin and other officers on his neck and back.

Eric Nelson, Mr Chauvin's defence attorney, has suggested at several points during the trial that Mr Chauvin's body weight was actually on Mr Floyd's shoulder or back at times and not on his neck.

Dr Tobin disagreed.

"Officer Chauvin's knee is virtually on the neck for the vast majority of the time," he said, "more than 90% of the time."

Dr Tobin is testifying as an expert witness for the prosecution.

The Chicago-based doctor said he has testified previously at medical malpractice trials but this is his first criminal trial and he is not being paid.

Derek Chauvin was fired from the Minneapolis Police Department

Mr Chauvin, who has pleaded not guilty, faces up to 40 years in prison if convicted of the most serious charge - second-degree murder.

The 19-year veteran of the Minneapolis Police Department was fired from the force after Mr Floyd's death.

Prosecutors are seeking to prove that Mr Floyd's death was due to asphyxiation, while Mr Chauvin's defence claims it was due to illegal drugs in his system and underlying health conditions.

Dr Tobin dismissed defence claims that any pre-existing medical conditions may have contributed to Mr Floyd's death.

"A healthy person subjected to what Mr Floyd was subjected to would have died as a result of what he was subjected to," he said.

Forensic scientist Breahna Giles testified yesterday that pills containing methamphetamine and fentanyl were found in Mr Floyd's car and in the police car, with some pills having saliva that matched Mr Floyd's DNA.

Several high-ranking Minneapolis police officers have testified that Dr Chauvin's use of force against Mr Floyd was excessive.

Minneapolis police chief Medaria Arradondo said on Monday that Mr Chauvin violated the department's training policies and its "values".

Mr Floyd was being arrested on suspicion of passing a counterfeit $20 bill in a nearby store.

A paramedic told the jury last week that Mr Floyd was already dead when an ambulance arrived and that Mr Chauvin was still kneeling on his neck.

Police officers are rarely convicted in the United States when charges are brought and a conviction on any of the counts against Mr Chauvin will require the jury to return a unanimous verdict.

Three other former police officers involved in the arrest are to be tried separately later this year.