Prosecutors are seeking stiffer-than-usual sentences for four former US police officers charged over the killing of George Floyd, arguing that they showed "particular cruelty" to the handcuffed African American.

The 46-year-old resident of Minneapolis, Minnesota died in May after being pinned to the pavement under the knee of white officer Derek Chauvin as Floyd gasped that he could not breathe.

Court papers filed Friday indicate that the Minnesota attorney general's office will argue there were a number of aggravating factors -- including that Chauvin knelt on Floyd's neck for almost nine minutes as bystanders, including multiple children, pleaded for his life and then watched him die.

"George Floyd, the victim, was particularly vulnerable because officers had already handcuffed him behind his back and then placed him chest down on the pavement, and Mr. Floyd clearly and repeatedly told the officers he could not breathe," the court documents state.

Chauvin inflicted "particular cruelty," as well as "gratuitous pain" as he abused his position of authority, prosecutors allege.

"Despite Mr. Floyd's pleas that he could not breathe and was going to die, as well as the pleas of eyewitnesses to get off Mr. Floyd and help him, (the) defendant and his co-defendants continued to restrain Mr. Floyd," the papers say.

Three or more suspects "actively participated" in the killing, prosecutors noted, saying this would justify longer sentences.

The charges against Chauvin include unintentional second-degree murder, while three ex-colleagues -- J. Kueng, Thomas Lane and Tou Thao -- are charged with aiding and abetting second-degree murder and manslaughter.

The shockingly public nature of Floyd's death -- which bystanders filmed and then posted on social media -- sparked an enormous mobilization nationwide, as protesters took to the streets to denounce racism and police brutality.

The face of Floyd, a father-of-three whose last job was as a security guard, has become a symbol brandished in anti-racist marches everywhere.

He was remembered at a massive demonstration in Washington on Friday that highlighted the case of 29-year-old African American Jacob Blake, who was gravely wounded when a policeman fired multiple shots at him as he tried to get into his car on Sunday in Kenosha, Wisconsin.

Conviction in Minnesota for second-degree unintentional murder and third-degree murder usually carries sentences of up to 12 and a half years.

The court documents did not say how much extra time prosecutors will request if the men are convicted, but the maximum for second-degree murder is 40 years in prison.

A defense attorney for ex-officer Kueng sought a dismissal Thursday, attributing the death to health problems and fentanyl in Floyd's system.

Attorney Thomas Plunkett will file evidence that Floyd swallowed drugs during a May 6, 2019, arrest for selling drugs and was convicted of a 2007 armed drug robbery in Texas.