Three police officers involved in the death of George Floyd have been arrested and charged with aiding and abetting murder.

The fourth former officer, Derek Chauvin, who pressed his knee into the African-American man’s neck, has had his third-degree murder charge upgraded to second-degree murder.

The upgraded charge carries a maximum sentence of 40 years, compared to 25 years for third-degree murder.

Mr Floyd’s family called it “a bittersweet moment” and said they were “deeply gratified” that Attorney General Keith Ellison took “decisive action”, a statement released by lawyer Benjamin Crump read.

The family has thanked supporters from across the US and and beyond for their efforts to secure justice.

“We are here proud that this family’s call for justice was heard by so many people not just in Minnesota, but in New York, in Houston, Texas, in Australia,” Mr Crump told reporters.

Quincy Mason, center, the son of George Floyd, and family attorney Ben Crump, left, kneel, as they visited the site of a memorial in Minneapolis. Photo: AAP

Chauvin continues to face a second-degree manslaughter charge.

The three other officers – Tou Thao, J. Alexander Kueng and Thomas Lane –  were charged with aiding and abetting second-degree murder and second-degree manslaughter.

The first count has a maximum possible sentence of 40 years in prison, while the manslaughter-related count has a 10-year maximum prison sentence.

The men had been stood down after the worldwide release of shocking video showing how Mr Floyd was pinned down even as he begged for his mum and told the police he could not breathe.

The officers had been called to a shop in Minneapolis to reports a shopper had used a fake $20 note.

Tweet from @AttorneyCrump

Minnesota attorney Jay Adkins was concerned Chauvin’s third-degree murder charge would have failed at trial.

“The third-degree murder charges…is invalid in this case because the officer’s actions were directed only to Mr Floyd not multiple people — like firing in a crowded building,” Mr Adkins told ABC News.

He cited a Minnesota Supreme Court case that reads “third-degree murder ‘cannot occur where the defendant’s actions were focused on a specific person.'”

Defence Secretary contradicts Donald Trump’s calls for military intervention

Widely seen bystander video showing Mr Floyd’s death has sparked sometimes violent protests across the US and around the world.

George Floyd pleads for his life before lapsing into unconsciousness from which he would never awake. Photo: CNN/Twitter

American President Donald Trump has threatened military intervention, after telling governors to toughen up against violent protesters, looters and rioters.

But on Thursday morning (Australian time) the president’s defence secretary Mark Esper said he is opposed to sending in troops.

“The option to use active duty forces in a law enforcement role should only be used as a matter of last resort and only in the most urgent and dire of situations. We are not in one of those situations now,” he told a news briefing on Wednesday.

Meanwhile, Governor Tim Walz and the Minnesota Department of Human Rights on Tuesday launched a civil rights investigation of the Minneapolis Police Department and its history of racial discrimination, in hopes of forcing widespread change.

The official autopsy by the county medical examiner concluded that Mr Floyd’s death was caused by cardiac arrest as police restrained him and compressed his neck.

A separate autopsy concluded he died of asphyxiation due to neck and back compression due to Chauvin’s knee on his neck and other responding officers’ knees in his back, which made it impossible for him to breathe.

-with AAP