The FBI will probe the death of an African-American man after shocking video showed him begging for his life while a police officer pinned him to the ground and pressed a knee into the back of the man’s neck.

The officer’s actions were slammed by Minneapolis Mayor Jacob Frey as “utterly messed up” and “wrong on every level”.

The policeman and three others involved in the arrest have since been fired, it was confirmed early Wednesday morning (Australian time).

In the 10-minute-long bystander video of the arrest, the man identified as George Floyd can be heard repeatedly saying “I can’t breathe” as he is face down and handcuffed on the ground.

“Don’t kill me!” the man yells before becoming silent and motionless.

The video is one of two to go viral in the US in one day as bystanders and victims look to prove how African-American people are treated by police and white civilians.

In a separate incident, a black American man bird-watching in New York’s Central Park had a verbal disagreement with a white woman who then called police and alleged she was being threatened.

Fears over the treatment of black Americans have been heightened in recent weeks following the shocking death of Ahmaud Arbery, a 25-year-old black man who was shot while out for his daily jog.

Minneapolis mayor Mr Frey said the officer “failed in the most basic, human sense” and apologised to the black community.

“For five minutes, we watched a white officer press his knee into a Black man’s neck. Five minutes. When you hear someone calling for help, you’re supposed to help,” he posted to Facebook.

Being black in America should not be a death sentence.”

The video shows he remained under the officer’s restraint, held down by the neck, for several minutes, despite explaining that he could not breathe.

Warning: This video contains graphic content 

Several witnesses gathered around the encounter can be heard demanding that the officer take his knee off Mr Floyd’s neck.

“His nose is bleeding”, one bystander says, as another urges the officer to “get off his neck”.

Mr Floyd was later taken to a local hospital where he died, the Minneapolis Police Department has said.

The officer was originally responding to a call from a local store that the man had used a forged check to pay for groceries.

Accompanied by another officer, they spotted the man in his car, whom they believed was possibly under the influence.

In a press conference, a Minneapolis police spokesman said the man “physically resisted officers”.

“Officers were able to get the suspect into handcuffs and realised that the suspect was suffering a medical distress,” the spokesman told reporters.

“Officers called for an ambulance.”