A four-year-old boy in the United States shot and killed a three-year-old in a shocking case that has rocked the Indianapolis community in which the tragedy took place.

Local policeman Lt Jeff Duhamell told The Indianapolis Star that the boy had picked up the weapon, a .45-calibre handgun, pointed it at the girl’s head and fired.

The gun had been left unattended on the kitchen table where the children were also left to play.

The girl, Aunesti Lee Allen, was not killed instantly but died several hours later at Riley Hospital for Children.

Aunesti was the daughter of the boy's father's girlfriend.

The girl’s mother, Fiona C Lee, has been charged with neglect and drug possession after police found cocaine and marijuana in the 26-year-olds house where the shooting took place.

Two young siblings of Aunesti who witnessed the tragedy are reportedly being cared for by a relative.