Police in the United States are investigating after a nine-year-old girl accidentally shot her gun instructor in Arizona.

The accident happened at a shooting range while the instructor was teaching the girl how to shoot an Uzi machine gun, according to a police report.

Charles Vacca, 39, was standing next to the girl when he instructed her to shoot a single round.

Then he switched the gun to the fully automatic mode, and the recoil caused the gun to swing around and shoot him in the head.

Mr Vacca was taken to a hospital but died a short time later.

A manager at the firing range told NBC News that firearms training there can begin as young as eight years old.

Firearms instruction at other shooting ranges in the US, which has the world's highest per capita number of guns, can start as early as seven years old.