Prosecutors have charged a Minneapolis police officer accused of pressing his knee against George Floyd’s neck with second-degree murder, and for the first time levelled charges against three other officers at the scene.

The new charges were filed by Minnesota attorney general Keith Ellison, who planned an announcement later on Wednesday.

His office did not respond to questions about the charges.

Widely seen bystander video showing African American George Floyd’s May 25 death has sparked protests nationwide and around the world.

Officer Derek Chauvin was fired on May 26 and initially charged with third-degree murder and second-degree manslaughter. The three other officers were also fired but were not immediately charged.

Mr Ellison upgraded the charge against Chauvin to unintentional second-degree murder.

He also charged Thomas Lane, J Kueng and Tou Thao with aiding and abetting second-degree murder and second-degree manslaughter.

Lawyer Earl Gray, who represents Lane, told The Associated Press that the Star Tribune’s initial report about the charges was accurate, before ending the call.

Before news of the upgraded charges, a lawyer for Chauvin said he was not making any statements at this time.

Lawyers for Thao and Kueng did not return messages seeking comment on the charges.

Benjamin Crump, a lawyer for Mr Floyd’s family, called it “a bittersweet moment” and “a significant step forward on the road to justice”.

Mr Crump said Mr Elison had told the family he would continue his investigation into Mr Floyd’s death and upgrade the charge to first-degree murder if warranted.

Mr Floyd’s family and protesters have repeatedly called for criminal charges against all four officers as well as more serious charges for Chauvin, who held his knee to Mr Floyd’s neck.

Some of the rockiness of the days since Mr Floyd’s death May 25 dissipated on Tuesday night, with demonstrations continuing around the country, but without major reports of violence.

Curfews and efforts by protesters to contain earlier flare-ups of lawlessness were credited with preventing more widespread damage to businesses in New York and other cities overnight.

Last night we took a step forward in moving out of this difficult period we’ve had the last few days and moving to a better time,” New York mayor Bill de Blasio said.

New York police said they arrested about 280 people on protest-related charges on Tuesday night, compared with 700 a day earlier.

Nationwide, the number arrested in connection to the unrest rose to more than 9,000.

At least 12 deaths have been reported, though the circumstances in many cases are still being sorted out.

Some tense incidents continued Tuesday night, but were far less prevalent than in preceding days.

Police and National Guard troops used tear gas, flash-bang grenades, nonlethal rounds and other means of dispersing crowds near a police precinct in Seattle, near Centennial Park in Atlanta and at demonstrations in Tampa and St Petersburg, Florida.

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