A rookie police officer in the US fatally shot a black 12-year-old boy within two seconds of jumping from his vehicle, a video released by police shows.

Tamir Rice died in hospital hours after he was shot twice in the torso from close range outside a recreation centre in Cleveland.

Security camera footage shows Tamir wandering alone between a sidewalk and a gazebo in a green area of the Ohio city, swinging the gun in his hands and occasionally pointing it while chatting on the phone.

Suddenly, a police car comes into the frame, pulling onto the grass a few steps away from the boy.

Two officers emerge from it and within 1.5-2 seconds Tamir is seen falling to the ground.

The video, which has no audio, was released by Cleveland police following a request from the boy's family.

"It is our belief that this situation could have been avoided and that Tamir should still be here with us," the family said in a statement released by attorneys.

"The video shows one thing distinctly: the police officers reacted quickly."

Tamir Rice shooting video released by Cleveland police IBTimes UK

Deputy police chief Edward Tomba said that, before to opening fire, the officers ordered Tamir to put up his hands three times.

In the grainy footage, the boy is seen reaching to his waistband before being shot and then bending over.

It was later revealed that he was playing with an airsoft gun that shoots non-lethal plastic pellets.

Police said the replica resembled a semi-automatic handgun and, crucially, had no orange safety indicator on the muzzle.

The two policemen involved in the incident, a first-year rookie named as Timothy Loehmann, 26, and a 10-year department veteran Frank Garmback, 46, have been placed on administrative leave.

Loehmann, who according to public records is white, has been identified as the shooter.

It was earlier revealed that a police dispatcher didn't relay information about Tamir's gun and age to the two officers.

The man who raised the alarm about the boy's presence told a 911 operator that there was a guy, "probably a juvenile", brandishing a pistol that was "probably fake".

However the team sent to the scene was simply told that there was "a male with a gun threatening people", the Cleveland Plain Dealer reported.

The video was released as the US has witnessed nationwide protests over another racially charged shooting.

Riots erupted in Missouri earlier this week after a grand jury decided not to charge a white policeman who shot dead Michael Brown, an unarmed black teenager, in Ferguson.